The more we love someone, the more that is on the line if that relationship were to fail. The more we let someone in to who we really are, the more fodder they have access to if they were to try to hurt us. The more a parent loves a child, the harder it will be when that child seeks independence. If an addict gives up his drug habit, it means he will be healthier and happier but it also means he may have to face the inner demons he was hiding by using.
Rabbi Marc Katz, in his Kol Nidre sermon about vulnerability. I thought the sermon was really fantastic and well-delivered. In fact, I strongly encourage you (Jewish or not) to read it; it has several good teachings and questions for us all to ponder.