The simple joy of teamwork. Nothing beats starting with fresh characters in a new world with 3 of your best buddies and taking out the Eye of Cthulhu or Skeletron with low-to-mid tier equipment or going on insane mining excavations searching for everything from glowing mushrooms to gold and jewels to Life Crystals all whilst tackling skeletons, bats, worms, fire imps, wasps, and maneaters, and doing it all together.
Screw teamwork. Any time you defeat one of the hardcore bosses solo, you'll feel like a badass. Especially the first time, when you don't yet have any of the top equipment that require the bosses' souls to craft.
Related is if you solo Ocram, the final boss of the console version.
Heck, soloing any of the Hardmode bosses is awesome. Particularly if you're taking another attempt at a boss that's beaten you many times before, only now you have that one piece of equipment that turns the tide. The Destroyer keep wiping you out? Well, as it turns out taking down Skeletron Prime first will net you what you need to make yourself a handy flamethrower...
Terraria seems to be created in mind of playing with friends, so playing on your own (freely or forcefully) might be some kind of Self-Imposed Challenge.
Come back to a normal world when you have that topequipment that requires bosses' souls to craft. Now try fighting the Eye of Cthulhu, something that likely gave you a lot of trouble when you first played. The battle will probably be over in less than 10 seconds. That's right. The basic hard-mode mooks are now a little tougher to kill than a body part of Cthulhu. Proving just how ultimately badass you are for conquering the end-game.
The 3 mechanical bosses can sometimes self-spawn at nightfallnote There are limits to this, the main one being that only the bosses you haven't beaten yet will get to do this, and their spawning can be cancelled if you spawn in a boss yourself during the minute or so between being notified of a self-spawn, and the boss appearing.. You get some warning and prep-time, but it might not be enough to get all the stuff you need AND get to the area you'd prepared for the fight against that particular boss. However, if you DO manage to fight the boss when they self-spawn, and win, you'll feel like even more of a badass.
Beat the Eater of Worlds, Skeletron, and the Eye of Cthulhu at the same time (Mind you, this was way back when those were the only bosses and they were hard as hell);
Defeated King Slime with a Copper Pickaxe, which only does 1 damage per hit to the boss, 2 if critical;
Has defeated the Brain of Cthulhu, Skeletron, the Eye and The Eater on the first day/night;
Has then fought all four bosses pre-1.1 during a Blood Moon with Battle Potions, and won;
Created an infinite water farm to refill his ocean (He decided it was a good idea to dump all of it in the Underworld)
Managed to defeat the Dungeon Guardian, Which is known to take only 1 damage per hit, has 9999 HP, and does over 1000 damage per hit; And he did it TWICE! (True the second time had the unfortunate two at a time Dungeon Guardian kill, but still...)
Beat all three 1.1 hardmode bosses at the same time, something he did without tricks (As in, heart statues)
He then defeated (He defeats a lot of bosses, doesn't he?) all pre-1.1 bosses in a Room of Death (As in, a room that has nothing but deadly spikes on its walls)
And if that wasn't enough, he defeated every pre-1.1 bosses using only explosives;
He also did a Night's Edge speedrun, an Excalibur Speedrun, which later became a Hallow Speedrun. Mind you, both of these were on Hardcore;
Then, after beating Skeletron on the first night, he entered the Dungeon with no armor, and came out of it with every item inside, with only 100 hp;
He also beat the Destroyer in 18 seconds using the Magical Harp, for a total of 4300 damage per second;
Then, he beat the Wall of Flesh with no armor, just a Water Bolt;
He also beat all 3 1.1 hardmode bosses with the Frost Legion as a bonus at the same time.
That's not all, He also beat TWO bosses on the same first night, Eye of Cthulhu and Eater of Worlds to be specific, and tried fighting the Wall of Flesh. From the beginning of the video, it made it clear that he just can't do that.
He then killed all the hardmode bosses with The Horseman's Blade.
He fought and won against eight Duke Fishrons at the same time.
He killed the Wall of Flesh on the Second Night, that being the earliest you can kill it.