I think that for some kids, that is very normal. And it is probably something that develops as they become more aware of others and emotions.
I know that Savannah started doing that (can't remember the exact age, but she's still very sensitive to others moods, although thank goodness Abe's crying doesn't seem to bother her at all or we'd be in trouble). I first noticed it when reading with Savannah--for example, she loved the book "Are you my Mother" but then one day, she just started wailing when the bird couldn't find his mother--she was so sad for him. She's done it with other characters too--she can't watch hardly any Disney movies because the whole mother issue makes her so sad and all those mom's die or something.
I don't think I'd worry too much about it unless you've noticed other changes too.
~Kim