All manner of reasons. Some legitimately felt called to serve. This is still a generation who's parents were drafted into World War II and Korea; in the psyche

All manner of reasons. Some legitimately felt called to serve. This is still a generation who's parents were drafted into World War II and Korea; in the psyche of a lot of Americans, the draft was still a virtuous thing that the well-chosen democratically-elected representatives of their great nation would never activate for a foolish boondoggle. They didn't know a Vietnam could happen because a Vietnam hadn't happened yet; they weren't the generation raised listening to CCR's "Fortunate Son." The pull of the mainstream is very strong. Five years in jail is also quite a long time, and as the war dragged on I suspect a lot of folks weren't sure how long those jail sentences would ultimately be; remember, if you were drafted, dodged, were caught, and served, you'd be immediately drafted again and could go right back to jail (because the war lasted longer than 5 years). Since the goals were unclear, I suspect many served thinking the war would be over soon and a couple years in a battlefield with "the army that saved the world from Hitler" would be better than cooling your heels in jail for five years (and then being branded a coward by your family the rest of your life because your dad lost two brothers in his War and you were too chickenshit to live up to the family legacy). Draft dodging wasn't the obvious correct choice until many years into the war; Americans had won (or at least come to a standstill on) drafted wars before.