it's debatable whether or not it's sadistik's answer or oshn's, because in the flashback thanksgiving episode, Chandler tells Monica to be a chef, and I think she responds with, "ok." but I'm not sure. At any rate, that's one reason.
The other comes in with TOW The Candy, when Chandler asks her, "Did you become a chef so people would like you?", and they hint around at that.
Whatever moonstrucksoul was going for, though
