What goes on in a boys head?

One minute, their all happy and friendly with you. The next minute, their all moody then start slagging you off. But why are they all the same? I like a boy, whos decent. Hes like that aswell. He likes to annoy me alot but i know its for a little giggle. I like to annoy him aswell. I thought that meant we were getting along. But yesterday he started on me for no reason. He started slagging me off infront of all his mates. Which made me a little embared and fustrated. I started to call him things back. Then he went in a huff with me!! I've tried phoning him and talking to him on msn, but he wont answer me. I told him its okay for him to slag me off but i cant slag him off? I didnt go half as far as he did. What goes on in a boys head? Because they really piss me off sometimes!!