Maybe Riker should have left, but I think he felt to attached to the Enterprise, nothing else was good enough. He even admitted it a few times (I think), and he was so dead set to be on the Enterprise that in the pilot (I think it was), it was mentioned he had a choice, CO of a starship, or XO of Enterprise, and he clearly picked XO of Enterprise. I agree though, they should have shown more deeper or personal reasons for saying, maybe a cool scene where someones like "Will, why are you really staying?" "Uh...I just..." *looks to Troi* "...feel it isn't the time to leave." Kind of corny and cheesy, I guess, but it works (somewhat, at least).