Sometimes what's best for you isn't best for others.
Sometimes you have to make sacrifices.
And sometimes the general concensus is, "why fix something that isn't broken?"
And that's why Megamind stared down at the white cape in his hands and let it slide from his grip and fall slowly to the ground, coiling there like molted snake's skin. That why he heaved a great sigh and swallowed hard, staring mel-on-cally down at the piece of fabric as the brainbots clipped his black cape back around his shoulders. Because he had to face the facts.
The world didn't need another superhero.
But it did need villains like him. Benevolent villains. Harmless villains. To keep the harmful ones away.
And that's why he walked past the cape and left it to rot, leaving his not-so-evil lair forever and turning his leaf back over. That's why he was the villain again. That's why he lost Roxanne. That's why his smile was always forced from then on. Because he'd had a taste of the life he wanted, and then grown up. And sometimes, when you grow up...
You learn to put other people's happiness above your own.