Embarrassment
The rate at which a person can mature is directly proportional to the embarrassment he can tolerate.
A simple compliment goes a really long way — for a guy to just come over and say, 'You have great hair' or 'I really like your dress', and then just smile and walk away. That's a great move, because he's sort of putting himself out there by doing that, but it won't lead to any embarrassment if the girl isn't interested.
Remembering that I'll be dead soon is the most important tool I've ever encountered to help me make the big choices in life. Because almost everything — all external expectations, all pride, all fear of embarrassment or failure — these things just fall away in the face of death, leaving only what is truly important.