There are myriad reasons why you might want to stop watching a particular anime. The show might have hundreds of episodes and you lose interest along the way (Naruto). It might get too weird and abstract (Evangelion). It might be overloaded with tedious filler episodes (Bleach). It might go on hiatus for too long and you move on to other shows (Fairy Tail). Heck, it might even get canceled while you're watching it (Hellsing).
This week's discussion question: What's an anime you started watching and then abandoned and why?