Now, either we're all just becoming really grumpy as we get older, or it may mean that as you understand laughter better, and you are getting better at doing that, you need more than just hearing people laugh to want to laugh.
Bem, ou ficamos mal-humorados com o passar dos anos, ou, talvez, quando entendemos  o riso melhor, e sejamos bons nisso, não seja bastante ouvir alguém rindo para querer rir.