Because I wanted to blow shit up...kill gomers and lay waste to their world.
Well...maybe only in part...but at 17 what the hell does any yout really know. In those days, my world then was rather unpleasant. I joined up to get three hots and a cot, learn a trade and play at war. Twenty eight years later, my missus declared my fun days to be at an end and I retired. I played at enlisted and officer. Both had their good points and both had their negatives.
It was the toughest life you can imagine...and the most stressful. It was also the most fun we have ever had. You NEVER forget those who watch your back...who share the same hole, eat the same chow...or dirt, are happy and scared like you...hate the same pompous asshole REMFs for the same reasons, love the same great commanders who you'd follow to the Gates of Hell itself. All these guys are the best...the men with whom you party until you simply can't stand...then fall down, get up and do it again!
I remember all those guys...the jerks and the heros. They shape us...they form our conciousness for the rest of our lives.
Go read Henry V. Shakespeare wrote a pretty good reason. Then go ask an old Vet to show you his scars on St. Crispian's Day.