Its because Bella has what Rosalie always wanted- a child. Bella was was human when Renesmee was conceived. Whereas Rosalie was turned before she had the chance to have a child and she uses Renesmee as a dream for something she never did. :)

Rosalie explains that this decision cancels out the earlier decisions she'd disagreed with, like becoming a vampire to be with Edward forever.

I don't know. I don't think Rosalie actually becomes Bella's "friend" in my opinion.