Because it's part of the universal law of attraction. It's like with anything that you hold on too tightly to. It's like watching water boil. When you sit there and wait for it to boil, it takes forever, but if you put it on to boil and leave, it seems like to boils way faster. We block energies when we hold onto them so tightly. We weren't meant to stop living our lives waiting on a person. They made their choice to leave in the first place. You process through the pain and then keep on living. They can feel when your energy is no longer with them and that's usually when they come back in. They feel something is "missing". But when we sit there and wait for them they can feel our energy still around them so it's like they won't get to experience a true "loss" if you will. Just because you aren't there in the flesh, you are still there in energy because we are all energy beings. That's why you will hear sooooooooooooooo many psychics tell you to "pull back your energy and let him/her come forward". Pulling back your energy means to focus your energy somewhere else besides on them. When you focus on them all the time they can feel it and have no reason to come forward cause they don't feel the lack or loss of you. Focus on you, work, school, hobbies, your life. That's called loving and caring for yourself. If you think about them here and there, even once a day, but then let the thoughts of them go.........that's called pulling your energy back and letting them go. Sometimes, when they leave us, what feels like a loss is really a gain.