What changed K's experience?

In MIB3, the proposed explanation for K's lack of emotion and human interaction can be traced to the happening at Cape Canaveral. When J goes back to change the otherwise altered past, K comes out more lively and joyful. As far as I know, though, the only thing that changed was that he killed Boris the Animal rather than arresting him, so what changed him so much for the better?

J's dad still died, if that isn't what devastated him, what did?

Best Answer

To me it didn't seem too much as he changed, he always cared about J and secretly took care of him like a father, K was there while J grew up and that's another reason he made sure the chosen agent in MIB1 was him. So it's more like when he finds out J discovered the truth he gets in a good mood (for being a quite stoic dude, he has his moments, specially with J) and they have a pie at the end of the film...