Disappearance.
When a person disappears, or leaves for a while, what do you do when they eventually come back? Do you fill them in on everything they may have missed? Do you try to ask them why they left in the first place? Then, there's also the situation where the person that disappeared does this kind of thing all the time and expects to come back without any consequence.
In that case, how long is too long? Days, Weeks, Months, Years? Does it affect how you view them when you see them again and they act as if they had never been gone? Would it hurt more if they were apologetic and wanted to know what had happened in your life while they ignored it?
What do you do if you never see them again? Were you the problem? Did something happen to make them runaway? There is grief in a loss, there is grief in reunion, and grief in the questions you have to ask yourself. Disappearance is not simple; there are too many variables to think about, so some simply don't.
This type of grief doesn't disappear.