lots lots of prepositions

I'm especially bad at using the right prepositions while at the same time the US population is using more and more of them. They are not only used everywhere but more often in pairs. "You are welcome to sit in on my class" rather than "sit in my class"; "Ha, you didn't pick up on that" instead of "you didn't pick it up"; and,... my roommate gave me this funny one:"Don't get on up in my face!" Heh. It's a trio. There are many more examples like those. Why do people do this though? I don't understand...