To be fair Neonivek does watch orders of magnitude worse anime than almost anyone else on this thread.
Kinda' makes you wonder if it's like a masochistic thing or something...
It is a combination of things
A) I don't want to watch something everyone else is watching... I know it is a negative trait.
B) If I watch a bit of a show or am enticed by the premise I am instantly invested
C) Good shows that I know are really good are intimidating because I feel obligated to like them. So I avoid them. Heck I forgot two REALLY good detective shows I wanted to watch because I avoided them too long.
D) I do honestly like some terrible anime. Master Cooking Boy was awsome just for that. The ultimate Anime Mary Sue who didn't arise from fanfiction (Mind you... Master Cooking Boy wasn't that bad. In fact it was probably good with just a lousy story). That and I have low standards in some areas.
E) I don't like things a lot of other people like. Familiar Zero I DESPISE and that Anime with Kyon, that I can never remember the name of, I also disliked greatly even if I liked a few episodes. Those animes are REALLY popular. Heck I don't even really like Bleach.
Ultimately it is because I can't really find animes I really like and some of the ones I do I avoid while at the same time I am easily invested and often ended up watching an anime to completion if only because I watched the first episode thus slowing down my search to a crawl.
If I wanted to watch more good anime I'd switch to Fairy Tail that I do like for some reason. A lot of people say they would love to be a member of Fairy Tale but I know myself well enough to just KNOW I'd just hate it.