People are as friendly as you are... obviously, use some common sense, not everyone in the world is nice, but if you run into a few, it doesnt make the entire city and its people bad... Smile! It makes a difference.
Of course that depends nieghborhood by nieghborhood. The best thing to do is find a home you like and then spend some time in that neighborhood before you are committed. Walk your dog (or borrow one, they are great conversation catalysts) around and talk with people. In very short order you will find out if you fit in. Like anyplace else, (but maybe a little more eclectic) LA has a neighborhood that will feel like home to you, you just have to find it.
With all places in the world there are those who tend to be bitter and have a poor disposition. Southern California is a melting pot of people from around the world (myself included: Kentucky) and I have met life long friends since I moved here in the 1980's.