Not all do lol. Some, many actually, are nicer to ones that they like. But to answer your question, it comes natural to some people. It's a weak attempt to hide that they like them, although it actually gives it away. Hope this helps!
i think its because guys don't know how to tell girls they like them. They'll grow out of it sooner or later but they like when girls say something first but don't ever cave. He has to be the bigger person
It's not "cool" to act nice. To guys, they seem to think it is unmanly to act nice to girls, especially if other guys are around. Plus they are afraid of being rejected. They do grow out of it, eventually.
maybe the fear of rejection. they are afraid you'll say no. they don't want to get embarrassed if you don't like them. i for one think its stupid.if they pretend to hate u u will hate them. and if u did like the person u will have second thoughts. so boys just tell girls u like them straight out it seriously helps more.