Look, I love living in New England. I was raised here, most of my family is from here, and I think it's a good place to be. But for most people, if you had to choose between southern California or Boston - especially if you're a black man given Boston's history - it's not remotely difficult to picture wanting to live in LA.
Add to that the fact that LA is a GREAT baseball city with a great baseball tradition, and their team is a huge market team that will consistently spend the $$ to put a good/great team on the field (not to say they won't have bad seasons, obviously), AND add to it that LA is, despite it being one of the great baseball cities in America, much more laid back and less of a pressure-cooker than Boston, and it's very easy to see why a guy like Mookie might prefer to be in LA. It's not like he signed in San Diego - another fabulous place to live but not really a city with a great baseball history. Or it's not like he signed with Pittsburgh, another city with a great baseball tradition but which isn't exactly a great climate.
Now I know he didn't orchestrate the trade to Los Angeles, but maybe if he got traded to Texas or Minnesota, he would have just become a free agent and signed with LA anyway. I don't know. But there's lots of reasons - despite our Boston bias - for people to legitimately want to be elsewhere. This place isn't for everyone, plain and simple.