How do you say goodbye to someone you were just starting to get to know? Does "I will miss you," even mean anything? Are these the hardest or easiest goodbyes? Hard because there's no certainty that a friendship will continue, hard because you're saying goodbye not just to a person, but to potential. Easy because their presence in your life was brief enough to be easily filled once its gone.
So, should you even say goodbye? Maybe "good luck" would be more appropriate, but those two simple words completely ignore what is being lost, even when you don't completely know what you're losing.