How about, to serve your country. You will also get excellent training which will help you in civilian life after you serve your term or retire. It teaches you responsibility, self reliance, discipline, pride for your country,and respect. The GI Bill, or what ever they call it now, is also a good reason. If you are on the ball, advancement through the ranks can be fairly quick.
The pay is (or at least wasn't) not all that great, the longer you stay and the greater ranks you achieve the better the pay. The benefits are great. Free health care and, if you are single you get free room and board.
I spent 7 years in the Air Force. The GI Bill paid for my college education. I could not have done that without that help.
Even if you just stay for your initial enlistment, you will be a better person that is better prepared for life and it's challenges.
Also, I grew up in the military. My father was in the Air Force and retired after 20+ years.