He's her Lobster!

PHOEBE: Hang in there, it's gonna happen.
ROSS: Wha, OK, now how do you know that?
PHOEBE: Because she's your lobster.
CHANDLER: Oh, she's goin' somewhere.
PHOEBE: C'mon you guys. It's a known fact that lobsters fall in love and mate for life. You know what, you can actually see old lobster couples walkin' around their tank, ya know, holding claws like...

Later, when Ross is trying to convince Rachel that they should be together--

ROSS: Uh, uh, well you're, umm, you're my lobster.
RACHEL: OK, you know what, are, are you being like, the blind date guy again?
ROSS: No no, you're uh, you're my lobster. See um, lobsters, uhh, in the tank when, when they're old, uhh, they get with, uhh, they walk around holding the claws. In the tank, ya know, with, with the holding and... Uhh, Pheebs you wanna help me out with the, the whole lobster thing?
PHOEBE: Do the claws again.

Later, after watching the infamous prom video, Rachel gives Ross a very passionate kiss and forgives him.

PHOEBE: See, he's her lobster!