Lol, because there will always be "friendly" State rivalry plain and simple
Angelinos don't hate New Yorkers--they just hate the success of all your sports teams. This is mostly because you are able to buy the better players due to the fact that your cable TV sales network makes more money than the all the other TV markets in the country do. This finances your teams to a greater extent than ours and everybody else in the same leagues with you and thus results in you being able to afford players even the people in LA can't (well, in general).
As far as New Yorkers themselves, we actually admire most of you guys and gals. You're fun, brash, and speak your mind (sometimes to an extent we wish we would do also). I've never found someone from New York who wouldn't just lay it all out on just how they feel about something. And I've found and associate with many wonderful people from the NY area who now live elsewhere--including my wife.
P.S. And you'll find that regional rivalries do exist everywhere also. People in the SF Bay area feel similarly about the LA sports teams, so SF people compete against ones from LA. Same with Boston and New York. It's more a miniscule sports rivalry thing than any real hate. Take Giuliani's recent comment as an example.
I'm from LA myself, although I now live in the SF area.
Enjoy and be cool
To be honest, New Yorkers aren't too bad.
I'd rather have Yankees fans visiting Anaheim Stadium to play the Angels than those obnoxious Boston Red Sox fans.