My first job ever was being a DJ for wedding receptions and similar party type events.
I got really into DJ-theory even though I don't know if I was ever especially good at any of it.
But needless to say, if your DJ is any good, he's not just picking songs at random - it is much much much more complicated and subtle than most people care to know.
Basically, DJing is the applied psychology of how people like to dance at parties.
How to get people dancing and how to keep them there.
It's crowd control via music and musical tastes.
My DJ-ness still effects how I listen to music. I can't just throw on a CD - I have to DJ for myself. Always 10 songs ahead, curving for subtle blend/changes in tempo, tone, instrumentation, etc.