If they have to leave you to realize they loved you, then they never really loved you in the beginning. They had to be without you in order to realize that they a good thing. However, the thing is that they shouldn’t have to leave in the first place to realize that you were what they wanted. The fact that they had to get away from you to love you is questionable.

While they’re searching for what they really want, who’s to say what they’ve done in that time of thinking. Possibly, they’ve hooked up with somebody, gone out and done some questionable things, or something that you wouldn’t have approved of. 

So, saying if they really love you, then they’ll come back is a total crock because if they loved you, then they would’ve realized it while they were with you, not without you. They shouldn’t have to leave and come back to realize that you’re the one for them.