A long time ago, when I was first starting to Roleplay, I settled on humans, because it required the least alien outlook to portray, I could just play... a guy. It helped me cut my teeth as a roleplayer.
Then it just kinda stuck. I usually build from a concept, and my first instinct to make it human. Only if the concept is really non-human will I start elsewhere.
Then in Pathfinder/3.x when humans started getting trait bonuses that made them attractive as PC race in their own right did I really start playing humans as a mechanical advantage.
Now I find that in Pathfinder, I have a hard time breaking loose from the bonus feat and extra skills.
But I have been playing a lot of dwarves lately, I like the flavor and background of them.
I will, however, never play an elf. Or half elf. I just can't stand them. And I don't think I've ever seen one played in such a way as to make me not contemplate elficide, even with a paladin. I don't know why, I guess it might be the dwarf blood.