Opening My Heart

I have a couple of patients at work that I am becoming very attached to. They are not healthy but in spite of that we have formed really good friendships. They are always interested in the kids, the husband, the house, and me. They always make my day better by making me smile and laugh. Losing either one of them would be like losing a family member. They are like a grandma and grandpa to me. Why do I allow my heart to open up to them when I understand what the outcome will be?