most series which I find interesting to begin with, start to bore me after 5-10 episodes. while they play in different surroundings, the patterns are usually the same. examples for this are eureka, fringe, house,ncis, warehouse 13 and so on.
if once in a while I find a series I really enjoy, they stop it

this happened with stargate universe and coupling for example.
also every now and then I watch reruns of older stuff like fawlty towers