i think that all of us are guilty of favortism in some sense somewhere downthe line.
I happen to like to train with a certain few people because i know that when we want to kick up the tempo a bit, i don't have to worry about going 'too fast' for them. I wont have to worry about being 'too careful' with them either. I expect to give it hard, and recieve it hard (keep your minds out of the gutter) when i practice.
But that only happens for a little while. I try to train with as many people as i can. As many different body types, and sexes as well. It is important to play with everyone in our dojo.