(I'm playing the devil's advocate here, trying to answer everything with generic answers and seeing if they're sufficiently powerful.)
Perhaps for the same reason you can get a tune stuck in your head, repeating and reinforcing itself. (For many people, including myself, the affect of "thinking a thought" is auditory - I hear myself say the thoughts I think.) Many kinds of thoughts, mental "tics", etc. may have this repetitive self-reinforcing pattern - not just self-hating ones. We may be seeing a natural selection effect where the most self-reinforcing thoughts last the longest - all due simply to the fact that thoughts can be self-reinforcing.
How are thoughts self-reinforcing? Thinking them (which we may process similarly to hearing someone else say them) creates emotions, and in some cases these emotions increase the chance of similar thoughts recurring, creating a positive feedback loop. For example, being praised or thinking self-praising thoughts makes you happier, while thinking self-hating (or other-hating) thoughts makes you sadder or angrier (with yourself).
This might be a contributing element in self-reinforcing feelings of malice, but I would expect that if you actually act on those feelings of malice (towards someone else), the results of the actual acts would swamp any such effects.
Does this sound possible?
I don't think I'm apt to amplify other sorts of thoughts, though I may repeat them.
The other day, someone did something I didn't expect. It was something many people have done before; something that I thought of as very normal, but that I in no way understood and had not predicted.
As I said, this had happened many time before, so I wrote it off as "me not understanding people" or "people are weird" for a second, like I usually do, before realizing that "bad at" really means "lacking basic knowledge", which I had never realized before.
And then I thought "I should ask someone who is different from me why people do that, and eventually someone will have an answer."
But many people will have many more questions like this. So, what have you observed people doing time and time again, but never understood? Or something that you only understood after a long time or asking someone about it?
And can Less Wrong tell us, not necessarily why (I for one can make up evolutionary psychology fairy tales all day if I want) but what conscious thought process occurs behind these events?