Dexter singing to Astor and Cody as they fall asleep/are asleep in episode 2, then being an understanding boyfriend to Rita about her hesitation towards sex.
Dexter: But it needs to be right for the both of us, or it won't be right for either of us. I don't want that, do you?
In season 1, the morning after Dexter and Rita have sex for the first time. This is the point where all his previous girlfriends left him, sensing something off about him during the act, but Rita doesn't. He's almost like a little boy when he asks in bewilderment "There'll be a next time?"
In season 2, even though Lila was obviously manipulative and crazy from the moment she showed up, the episode where she managed to talk Dexter down from killing his mother's murderer, thinking that he is using heroin when we the audience know what his real "addiction" is. Dexter comes back to the hotel, bloody and distraught, and starts to tell Lila that he didn't do what he was about to do, and she responds with "I don't care what you did" and just holds him until morning.
Angel being unresentful of Dexter or anyone else for Lila's framing of him for rape. So much so it makes hardened sociopath Dexter tell Angel that "If he could anyone, he'd be him".
Dexter not wanting to touch Rita's stomach during the couple's yoga class when the fathers were supposed to "send their positive feelings" to the baby, then apologizing through internal monologue and revealing that he just didn't want to hurt the baby.
This wouldn't count in any other show but: "Nobody hurts my children."
Dexter's talk with Spirit Harry in the last episode of season 3.
Episode five, when Dexter rushes to the hospital after Rita has a pregnancy scare. The piano playing in the background makes the scene a combination of Heartwarming and TearJerker.
Rita: I don't know...what I would have done if I'd lost the baby and you weren't here. And nobody could find you.
Dexter: I'm here now. I'm not going anywhere.
Dexter conversation with Ramon Prado. He intended to show the mourning Ramon proof that Miguel was a murderer but as the conversation went on, he decided to "soothe the wild beast by removing its thorn." He realised that Ramon was just a victim of Miguel, who'd given his whole life to help his ungrateful brother, and that now Ramon was trying to change his life and help his children, showing strength and selflessness Miguel never had.
Deb being promoted to detective.
Dexter's proposal to Rita and the kids, which sounded sincere for him.
Dexter's reaction to Debra being shot in season 4.
Cody interjecting he's thankful for Dexter after the Mitchell family's slipping mask of wholesomeness got shattered against a wall when no one was thankful for Arthur at their Thanksgiving dinner.
Deb telling Dexter he's been the one consistently good thing in her life in the season four finale.
Harry's So Proud of You after Dexter's beats up Olivia's stepfather, followed by Dexter and Astor mending fences.
And Dexter telling Astor that he loved her.
In the season 5 finale, Lumen smiling for the first time after dumping Jordan's body, having finished her revenge and not feeling her Dark Passenger anymore.
Angel being perfectly okay with Deb being promoted over him, and just being happy for her.
Dexter's relief at Harrison's appendix being successfully removed, even hugging the doctor.
After a whole episode of Dexter's dark passenger taking the form of Brian and urging him to kill anyone he feels like, Dexter realizes that he truly doesn't want to, hits the imaginary Brian with his car, and picks up a hitchhiking Harry as he enters Miami. "Welcome home, son."
In season 6, right before Harrison and Dexter are about to go on stage as the lions from Noah’s ark, Dexter says, ‘We've got an ark to get on, and that boat has room for all the animals. Not just the sheep and the zebras, but the lions too. And Dexter is a lion.’ After you consider the final scene in season 1, in which Dexter actually showed how he longed to fit in and be open, and after his agonising soul-searching throughout season 6, it’s truly heartwarming to see he’s started finding his place. Later, when he kills Travis, he gives a speech, saying ‘Light cannot exist without darkness. Each has its purpose. And if there is a purpose to my darkness maybe it’s to bring some balance to the world. Because, let’s face it, the world is going to be a better place without you.’ If you’ve managed to really relate to Dexter so far, this scene would probably bring you some sweet, sweet catharsis.
Quinn telling Deb she forced him to take a hard look at himself, which made him a better person.
Dexter acting like any addict who wants to recover after Deb discovers his secret. He actually makes himself call her shortly before he was going to give Lewis his usual treatment, asking her to stop him, and she replies that this shows he can change.
In the season finale, when Dexter is arrested, Angel never stops defending him from the time he arrives at the station until the end of his interrogation.