I joined for several reasons:
I decided to go to a military school for the discipline in study that would be impressed on my very undisciplined self in hopes of getting into medical school. I planned to go into the Air Force as a physician. First, because it has always been a dream to go into the military and because I have felt God's calling to defend our nation. Also, because they pay for medical school (over $150k bill) and physicians do not have to pay for malpractice insurance (another very expensive bill for civilian physicians). When I did not get into medical school despite having the credentials, I felt God's call to the military despite feeling His closing of the door to medical school.
Despite my eyes being bad and having no flying experience, I was told that I could fly in the USAF as a navigator. I prayed and God gave me peace with the decision to join. I wanted to help out my fellow Americans. Once getting in, I was given all of my first choices of assignments. I chose the B-52 because of the importance of the navigator position on the aircraft and because of the long range of the bomber which would allow me to be at home more with my family.
So, I felt God's call, had had the desire to fly since I was young (I found a paper I wrote in 3rd grade that told what I wanted to do when I grew up and guess what? It said fly in the Air Force.

), and because it was a good stable job that would fit with my newly disciplined self.
The benefits are also a reason. Besides the job security and insurance, I am getting a Seminary degree for free while I am in and I will later get another masters, also for free. The retirement of 50% of your base salary for the rest of your life after serving for 20 years is also a huge plus ($50k/ year (for a retired Lt. Colonel) with yearly increases after retirement? Ok!)
I hope this helps!
God bless,
Dave (1Lt USAF/ B-52 Stratofortress navigator)