Memory, Feelings, Change

Augustine -- "...the remembering is far different than the experiencing. Without rejoicing, I remember the joys I once felt. Without renewed sorrow, I recollect past sorrows. I can review old fears without being afraid and remember what I once wanted without desire. In reflecting on some memories, I now feel the opposite emotion I felt then" (p. 151 , The Confessions of St. Augustine, Modern English Version, abbreviated).

I'm glad certain feelings won't last forever... Why is it that feelings can be so piercing in certain areas, and in regard to other aspects be almost numbed?