Why does Mike go back to Vietnam to find Nick?

Rating: 8/10 (6 December 2014)

In the first part of the movie, Mike and Nick are at Steve's wedding. They are both drunk and stumbling through the streets when Nick suddenly tells Mike not to leave him in Vietnam. This foreshadows the idea that Nick wont be leaving Vietnam. Throughout the first part of the movie, Mike is sort of the strong, responsible father figure to his group of friends, seen when he taught Stan a lesson in responsibility when he forgot his boots. It makes sense then that Nick trusts Mike to make sure he gets out of Vietnam, back to the home they both love.
So when Mike returns home from Vietnam, after seeing Nick get lost in the Russian Roulette gambling scene in Saigon, he is welcomed home by Linda. They are both mourning the loss of Nick, and find comfort in each other. But when Mike finds Steve in the Veterans Hospital with lots of money in his sock drawer from Saigon, he knows Nick is still alive. He remembers the promise he made after the wedding, and realizes he broke the one pact he made with his best friend. So Mike goes back to Vietnam to try to get his friend out of there.
The driving force behind Mikes attempted rescue mission is his promise to Nick. Nick from the start knew he might not be strong enough to make it back home without the help from his best friend. Mike also knew that Nick must have been changed by the war experiences they went through, when he didn't come back to the home he loved so much. Mike realized he let his best friend down when he left him in Saigon, and was willing to bring him back at all costs. Even when Nick didn't recognize him, Mike was still trying to keep his promise, and even risked his own life to try to fulfill it.