An Empathizing Kid

Recently, Kiki gets emotional (sad) and starts to cry quietly when she hears a sad sounding lullaby.
I was quite taken aback and don't quite know how to react to that. (I told her; "In life, there are good and bad things, and it's okay") Even when a cartoon duck cries (on TV), it took a short while for Kiki to empathize with it and cried along. Gosh, why is this happening? I hope it's just a developmental stage. I must go read this up from the net...