Believe it or not, most of them tend to enjoy battling and becoming stronger. They seem to have a natural drive for it. Just why that is, I'm not sure. Every time I've asked the Pokemon I have that can speak, she's just been confused as to why I would think they don't. Certainly, some Pokemon are less aggressive and eager to fight than others, but on the whole they seem to enjoy battling.

What they gain from it is experience and growth, new abilities, and sometimes changing into another form. It's an odd method of evolution and building up of strength, but that seems to be how things go around here.