Gump is giving the correct advice, sir. As you are, too. The fact is, he can date and all. You are advising him to live his life, which is exactly what he would be doing. If she decides to come back in the future, then he will cross that bridge at that time. But he can't just sit around and not enjoy someone's company in *hopes* of her coming back. Who knows, maybe the real one would come along. Not busting your chops, just giving a different viewpoint.
There are moments in this life when you are so confident in the rightness of your actions, that not even for a second do you consider the option that you might be wrong.
