There once was a character who is so heroic and kind-hearted that this instantly draws others to him, often making them a loyal band of life-long companions. This character has given love and support for his family members. This character has made the worst of enemies into the best of friends, either by circumstance or by force. This character always keeps the band together in perilous times and always sticks close to his loved ones through the good times and the bad times. This character is always sociable to just about everyone they meet.
Then a few years have passed by and the character hasn't really kept in contact with his family and friends. He doesn't take any opportunities to visit them. He may not even bother to pick up the phone and call them or answer their calls. He even doesn't say "hi" to the people he passes by, let alone socialize about things he isn't interested in. This is the implication that the character has turned from sociable and outgoing to introverted and distant.
There may be a couple of reasons for the character's current lack of communication with people:
Deceased loved ones: The character may be going through a difficult phase. Perhaps he did keep in contact with his friends, but somehow, some villain took his relatives' lives away, and this greatly traumatizes him that he doesn't bother to be approachable to others.
Business before pleasure: The character may be in the workforce and may be more focused on his earning a check and working hard more than chatting with his friends, not to mention the character isn't sociable with his co-workers. Remember, Being Personal Isn't Professional.
Heavy bullying: Perhaps the character wanted to blend in with their peers (although in some other reasons, s/he is expected to socialize with them). Unfortunately, the character is rejected and is the main target of their bullying. This goes with the Armor-Piercing Question what's the point of socializing if there are people who won't accept them?
The character Joaquin Phoenix does this in I'm Still Here when he ends his acting career to become a hip-hop star, cutting himself off from his former life.
Adam Sandler's character in Reign Over Me becomes this after his family is killed in the 9-11 attacks.
Don Vito Corleone chides mortician Amerigo Bonasera for this type of behaviour in the opening scene of The Godfather after Bonasera approaches Corleone for help when the court — and justice — fails his daughter and his family.
In 10 Things I Hate About You, Bianca mentions that her older sister, Kat, used to be very popular at school until "she got bored of it or something". It's later revealed that after sleeping with her boyfriend under the rationale that other people were doing it, deciding she didn't want to sleep with him again, and then being hurt when he dumped her as a result, Kat resolved to never again do things just because other people expected it of her and became viewed as "the Shrew" as a result.
In the Circle of Magic book The Will of the Empress, this is one of the main points of conflict. Foster-siblings Sandry, Briar, Daja, and Tris have been physically and emotionally distant from each other for several years, and Sandry in particular harbors a great deal of resentment for the fact that her duties forced her to stay in Emelan while the rest of them got to travel.
From the New World does this with Shun, who distances himself from his closest friends and even moves out of his hometown because he's turning into a Karma Demon.
In Island in the Sea of Time, Marian Alston cut off all ties with her kids after she realized that she was a lesbian and divorced her husband, because her husband threatened to out her to the Coast Guard unless she gave him full custody of the kids and agreed to stay away. Since then, she doesn't socialize much because she's afraid one of her subordinates will learn her secret.
In Bakemonogatari, Senjogahara was very friendly and social during middle school, but after having her weight, and emotional attachments, taken away by a crab-deity, she becomes totally isolated.
An episode of Law & Order: Special Victims Unit had a woman who had been raped by the same man on numerous occasions become this. After the first time, he was never caught and eventually tracked her down again. Even after changing her identity, moving to another state, and becoming a paranoid shut-in, he still found her and did it again.
This happens to Scully on The X-Files as a result of the intensely insulating nature of her work. Mulder was introduced from the beginning as a loner with few friends, made worse (or even caused by) his life-time devotion to finding his sister. Scully, on the other hand, was social in the early seasons, enjoying spending time with her friends, her godson, her parents and her siblings, and even has a date or two. She mentions in season one that she would like to "have a life" and that she doesn't see herself as immersed in the work as Mulder is. Fast forward a few years, and we see her lose touch with friends and pull away from her family. Her brother Bill notices and becomes increasingly annoyed when she visits him for Christmas and rather than spend time with him, his wife, and her mother, Scully becomes involved in an X-File. She does maintain contact with her mother, but even her mother expresses worry that she doesn't share anything with her and instead "keeps everything so bottled up."
The Doctor in Doctor Who is normally a very outgoing and friendly type, but after being separated forever from his companions Amy and Rory, he has a bit of a sulk, parks his TARDIS on top of a cloud, and becomes a bit of a hermit for about half an episode.
Monk was this before Trudy died, despite still being a neurotic mess he was still functional enough to still do active police work. After her dead he suffers a complete breakdown.
In Timon of Athens Timon starts out as the friendliest, most loved man in Athens. Then his creditors take all his stuff and his "friends" won't help him out. He becomes a misanthropic hermit who just wants to be left alone.
Both the Warden and Hawke in Dragon Age: Origins and Dragon Age II, respectively, show the tendency to leave behind their Fire-Forged Friends at the end of their adventures—a fact that is lampshaded at the end of the second game, as one of the Warden's True Companions is now desperately seeking to reestablish contact with him/her.
At the end of Final Fantasy IV, Kain Highwind refuses to attend the reunions and celebrations and goes to Mount Ordeals instead to atone for his actions. He stayed there until the sequel, which takes place seventeen years later.
The whole cast of Final Fantasy VI counts as this. Halfway through the game they get flung to different areas of the world and while that happened involuntarily, they only reunite when one party makes the effort to find them all a year later.
In Sonic Chronicles, most of Sonic's friends were a bit resentful towards Sonic because he didn't keep in contact with them for two years after defeating Dr. Eggman together — particularly, Knuckles had called him out for this and when Sonic asks Tails about how he felt, he didn't want to talk about it, although he still felt disappointed in him.
Erfworld has Charlie, who used to attempt to conquor all opposition, and rule the world as one side, who suddenly went in the complete opposite direction after attuning to the Arkendish. He has one city, and seems to allow noone to see him.
Zeta (Zimmy) from Gunnerkrigg Court knows that she's dangerous to anyone around her and so usually tries to stay away from everyone except Gamma, who acts as a human power regulator for Zimmy.
Sir Isaac Newton was known for this despite being an extremely respected member of the scientific community. On one occasion, he even went as far as to write to everyone he knew and basically say he wouldn't contact them again (he was back in touch within a year).
Actor Jeremy Brett was naturally quite outgoing, but had to adopt an introspective and sometimes antisocial persona in his portrayal of Sherlock Holmes - and in the process, to some extent Became The Mask.
Syd Barrett of Pink Floyd during his early career was said to be pretty outgoing. By 1975, due to a combination of drug abuse and pressure from the limelight, he had shut himself off from everybody but his family, wanting nothing more to do with his musical career, focusing his time instead on private ventures such as painting, gardening and do-it-yourself projects.