A relatively well known phenomenon. I get misty eyed now at all sorts of things I can't explain. Much of it is based in art. WWII movies which I've seen 100 times like saving private ryan when he's standing next to the grave or music about raising kids and then thinking about my own children (Cat Stevens-Father and Son, Harry Chapin's Cats in the Cradle).
I think it stems from years of trying to be the tough, hard ass society makes us think we need to be. Also comes from aging. That fvcker, Father Time, is starting to get the best of me. I think we see the innocence in us.