Why do we sometimes habituate to things like the flushing toilet? We take it for granted, but in other cases, your spouse has an annoying habit. And every time they say or do something, it irritates you even more than it did before. Why do sometimes we habituate and sometimes we sensitize? It's a mystery though. I don't think it's known. If you want to look at an evolutionary argument, it might be that we would habituate to good things but be sensitized to bad things.