besides for the fact that tPTB obviously don't want them together (did you see the teaser for next week1!!) Rachel wants better than Joey. Yes, while J is sensitive and puppy dog cute he's also [excuse me for saying this] not the brightest or the one with the best career aspects for the future!! WHat i'm saying is that he doesn't bring much to the table, while he may be adorable. rachel is smart and deep- she wants more for her life besides puppy dog adorable.
not that that means ross, but who knows. i've always maintained that chandler was the best catch in all aspects . mon got lucky.