Adrenaline, cockiness, alpha male syndrome, mental illness, desire to do what they feel is best for their country. It can run the gamut of reasons and not all are bad.
Its not a game though - video game wars are fun. In reality people are dying. My ex husband (Command Sgt Major in Army) watched one of his soldiers die in his job (EOD - Explosive Ordinance). His duty was to go along the roads in Baghdad and look for the IED's and take care of them. One exploded and he lived for 20 min with a big hole in the back of his head while begging my ex to get his girlfriend.
While the dangerous jobs may sound glamorous, remember its deadly and NOT Hollywood. I wish you the best.