I'm afraid the title isn't very good, as I had trouble condensing what I remember. The book starts with a man going out and getting drunk with a friend, and he wakes to find that he signed up for the military the night before and is expected to report for duty soon. He tries to get out of it but can't, so finally gives in and reports for duty.
At some point, I can't remember when, one of the people he meets is a seven year old girl. He kind of falls in love with her at that point, although he doesn't realize it. When he returns from going to war on a near-lightspeed ship, she is nearly his age due to time dilation, and they get together at that point. That part really stuck in my mind because it was so unusual. I've been trying to think of the name of this book for a while, and just realized that I could ask here.