Yes, boredom and suckitude (totally a word), or to put it in more collective terms a failure to engage; I'll try to give most films a chance ensnare my interest in some way, even a terrible film can be unintentionally funny, which can at least make the experience entertaining, but if you're film is really offering nothing for me that would make sitting through it worth my time (a couple of examples being the dreary, monotonous and pretentious Gerry and The Ring, although calm down I found the former to be exceedingly worse in almost every respect) then you've commited the cardinal sin in my book. Really that's about it, I'm totally desensitised so the likes of 'A Serbian Film' (or really, anyhting that's gratuitous violence/gore for the sake of it) really don't phase me, it just seems rather jejune. >.> So unless I encounter something that's just disgustingly bigoted to the point that I no longer feel like drudging through such dross, then most likely my sole reason for dropping out of watching something mid-way is simply a failure to engage me.
