The way I try to look at it, even though it doesn't completely take the hurt away, is similar to that saying about "some people enter our lives.......leave footprints on our hearts..."

The ones who teach us the most or help us grow the most seem to be the ones to leave us because we subconsciously drew them to us when we needed them most and when we were open to their help. When they help us with our struggles to the point that we can help ourselves through the things they taught us, we no longer "need" them. We have to let them go so they can help another person that is where we were before we met them.

Just because they were only there with us in physical form for a short while, we don't have to let go of that spirit.