Why was Sean Divine embarrassed with his friends in Mystic River? He acted as if he didn't know any of his childhood friends, Jimmy and Dave, to his partner although he had cared about his friends all the time. Sean also tries to pretend like they were never close at any time in his life. Why did Sean behave like that to his partner?
As I see it, there are quite a few reasons for Sean's behavior.
First of all, I think that the neighborhood they came from was a labeled as a working class neighborhood, and he wasn't very proud of it - especially when he was trying to build his reputation as a detective. On the other hand, you shouldn't forget that the horrible event which he witnessed in his childhood haunted him for the rest of his life - he and Jimmy tried to basically put this "away" and think about it less as possible, each of them in his own way. As they said - this easily could've been anyone of them, and this thought literally haunted them.
I think that this general "condition" led to this approach. He probably lost contact, intentionally, with all of his childhood friends and as the years passed by, they just grew apart - especially when he became a Massachusetts State Police detective and tried to earn reputation and respect within the ranks.
I don't think that this situation is so rare in life - some people, especially those who are trying to "escape"/forget their past (even if it's not horrible) and generally "change the scenery" of their lives, almost always grow apart from their childhood acquaintances, which result a strange behavior, from both sides.
The childhood abduction and the murder investigation in Mystic River take place 28 years apart, and we don't really know at what point in that gap Sean grew apart from Jimmy and Dave. I was left with the impression that Sean hadn't spoken to either of the other two men in many years, maybe the majority of that time. It could simply be that Sean legitimately did not feel close to his old friends, having moved on in his life.
There are a few other reasons that Sean might have wanted to distance himself from his friends, including Jimmy's past criminal record. It might not look good for a police detective to be long-time friends with a felon. It could also be that Sean felt a little bit of guilt about Dave's abduction, so he wanted to emotionally disassociate himself with the people involved in that incident.
Finally, I think the biggest reason that Sean had for wanting to deny his childhood friendships was a professional one. His partner reminds Sean a couple of times during the investigation that he (Sean) is too close to the people involved with the murder to be objective. He might fear that he'll be removed from the case if he admits to a close, personal bond with the father of the victim and (later) one of the suspects in the case.