Nothing, really. A lot of entertainment (not just anime) in general has a lot of changes in moods throughout. A lot of people like the change of mood because sometimes, it gives the viewer a sense of fresh material when they see something light-hearted in contrast to a super-serious fight or whatever.
Some people now prefers like Ecchi, Romance, Harem and all of that, like High School DxD and HOTD has too many fanservice, i started reading the manga of the series, and it doesnt have nothing of fanservice, IMO i prefer read the mangas to know more the stories and stuff, some animes finishes happy or not, depends if is a continuation final, just Date a Live, also for Elfen Lied they said that there will be a continuation, sadly cancelled, I think is for the fanservice, i mean Lucy at the final dies for real, so that's the problem, there are happy finals and sad finals, and those finals are different, Bokurano was even depressing the final but it was awesome, so there are differences in the animes, everything depends the genre like if is Comedy, Romance, Shonen, Ecchi, Gore, ect.
