I have a few reasons... right now, I'm active, but these are why I might stop at any given time.
1.) I can't keep myself motivated on my own. I can't reach a co-author right now, so I'm just... :P sometimes.
2.) Distractions. BoredPanda, this website, Pokécharms, everything...
3.) Every character is always bad if I make a new one. I'm constantly improving, but I'm PRETTY SURE I'm almost perfect by how much I criticize myself.