Simple answer, they grow up and see the truly important things in life. typical young men are still infused with this need to have sex with everything in motion, and OFTEN. They also take little value into their future and the need to settle down and grow up. They are so en grained with trying to prove that they are still as sexy, cool, athletic as they were in high school. They are unconcerned with compassion, labor, or giving more than receiving. Boys try to embody the men they grew to love, like football stars, or handsome uncles, when in reality, they need to become their own man and pave their own path. Making your own way is the most exciting part of growing up. The beauty of the United States is you CAN be whatever you want to be. That is a beautiful thing once it is seen. Instead of proving yourself to your guy friends, why don't you look bigger to the world?
But whatever it is that the military or religious establishments do to their men...It is good. They come back with a tremendous amount of respect for everything in their path. Respect for life, Respect for love, Respect for work, everything. I imagine church and the military have a lot in common, I imagine one thing you learn in both places, is humility. Being humble calms a man down quite a bit. Another thing is guilt. People who feel guilt and remorse for past mistakes are more likely to try to do better and be a better person, if you have no regrets and no guilt it is possible you are doomed to repeat the same mistakes. .....my rant of the day.