Why did Mike go back to Vietnam?

Rating: 9/10 (13 January 2012)

Mike went back to Vietnam in order to retrieve Nick. He felt that it was his duty, because he had promised Nick that he wouldn't leave him in Vietnam. He even felt like he couldn't go on with his life, because he hadn't fulfilled his promise. He didn't go to his own welcome home party, because he felt like he didn't deserve to be home. That's the same reason he wouldn't get into a relationship with Linda. He felt like since he left Nick in Vietnam, he couldn't be happy, and go on with his life. He's the kind of guy who always does things in order. He shows that in the fact that earlier in the movie, before the wedding, he berates Nick for not having shined his shoes in advance. He always keeps his promises, and he can't live with himself if he doesn't. So he goes back to rescue Nick.