mostly for the same reasons as the cast
Matt LeBlanc: It felt wildly inappropriate. That’s how close we all were to the character. I was like, “That’s Rachel. She was supposed to be with Ross. Wait a minute.” Everybody got super-defensive about the whole thing. We went to David and Marta as a group and said, “We’re really concerned about this. It doesn’t feel right. We have a problem with it. David said, “It’s like playing with fire, and then you put it down, and you go, ‘Remember when we played with that fire?’ We’re aware of everything. The feelings that you’re feeling, we’re feeling them, too, and we like it.”
David Crane: In [season 8] we had Joey fall for Rachel, and that scared everybody. She was pregnant. The actors freaked out. Matt kept saying, “It’s wrong, It’s like I want to be with my sister.” We said, “Yes, it’s absolutely wrong. That’s why we have to do it.” You can’t just keep spinning the same plates. You have to go places where you’re not expected to go.
Marta Kauffman: Everybody knew where we were going to end up. Ross and Rachel were going to be together somehow. We just had to make it entertaining.