Why aren't Scott Disick and Rob Kardashian friends anymore?

i remember they use to be tight them getting drunk in Vegas together
was so funny, but they don't anymore on the show and on the preview of
Keeping Up with the Kardashians (
http://www.youtube.com/watch?v=3ozRdIoZTlg ) it shows Rob saying that
he can't stand Scott

so does anybody know what happened between them?