How do I cope with a good friend joining the military?

My best friend since high school has always supported me in all aspects of my life. We have always sort of thrived on our differences. For example, I'm a scrawny, extremely focused music student, and he's an athletic, eclectically interested scholar of law, literature, and and various dead languages. It gives us plenty to talk about. I'm concerned because recently he has been feeling like his life could use more direction. As long as I have known him he has wanted to be a Marine sniper, and while to say the least he has taken a fairly circuitous route toward achieving that goal, I have no doubt that if tomorrow he decides to join the Marines, he would be more than capable of becoming the sniper he has always wanted to be. Without going into details, my career has been very precocious and promising, and his constant support of my dreams has meant everything to me over the past several years. I wouldn't be much of a friend if I didn't, in turn, support him in his own lifelong pursuit. That said, the thought of him in combat situations keeps me up at night. I'm worried that even if he's deployed and returns physically unscathed, he will be a completely different person from the buddy I became so close with. How do I, for lack of a better phrase, suck it up and deal with these concerns on my own, so that when the day inevitably comes when he tells me he has finally enlisted, I can share in his excitement the way he has so often shared in mine? Thank you for your advice.