Everybody gave out valentines because of this pressure. The reason our teachers did this was so nobody would feel left out, but we always found ways to show our crushes how we felt by putting an extra little heart on it or giving him or her the best one. This could be a reason why some people have soured to the day, an early life lesson in rejection. Of course, it was also a competition to see who had the best valentine back in the day. It was a tight race between those holding the little candy hearts with messages and the Lifesaver cards.
Some individuals still do take the time to send out cards and buy little trinkets for their buddies.
It's almost a mix of habit and boredom. The problem with sending valentines after we become a certain age is that we have enough to worry about with our busy lives, and our own significant others. Many people stop giving out valentines after grade school, and especially by high school. Unfortunately it is around this time that we need the most assurance that we are cared for and thought about. Saying these things is certainly not immature, but maybe sending corny little cards is.