Boy, yeah, I have that problem. In my undergrad, there was a teacher I really respected, and I really wanted to get to know him, but I always felt like such a waste of his time. Happens with most older guys I really respect who don't go out of their way to befriend me.

However, if it's a matter of professional self interest, there's no problem. I'm quite bold going up and talking to people very senior in rank.