The idea of the two of them is good in the beginning, seeing as Ross always had feelings for Rachel and they're both at points in their lives where they can be together. Their relationship goes well at first, but just seems to snowball. It's apparent that they were always supposed to be "endgame" but there just seems to be a point where enough should be enough. How many times can a relationship fail before you call a quits?
They were probably widely liked at the time, and keeping them together and dangling them on this string of near separation probably did great for the ratings of the show. But, sometimes, it's better for a break to be a break-up.