Colonel Trautman bailed Rambo out of his personal war against the police in the first movie. In the third movie, John jumps straight into bailing Trautman after learning he got captured by soviets, because Trautman would have done it for him.
Rambo really hates it when P.O.W.s in Vietnam were just gonna be abandoned. He went through Vietnam and tore the enemy bases to shreds just to rescue every single POW he could find, and then proceeds to scare Murdoch to find all of them.
The following scene has Rambo telling the Colonel that despite earning another medal of honor, he wants it to be passed onto the P.O.W.s he rescued, calling them the real heroes.
Then when Trautman affirms with Rambo about his hatred towards the United States, he immediately says this:
Rambo: Hate? I love this country. I want this country to love [the P.O.W.s] as much as they love this country!
One interpretation of the following dialogue is that Rambo has absolutely no regrets in coming to his former commanding officer's rescue, and ensures he never has to say it twice.
Trautman: John. I'm sorry.
Rambo:No you're not. *proceeds to fight along side*
Fourth film: After so many years of suffering, pain, and fighting his own demons, John has found peace, and, at long last, finally comes home.
Most of the interaction between John and Sarah. She's the only one to be nice to him, and he's only "nice" to her. She was even the one to convince him to go home.