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.
要么是我们的脾气随着自身衰老变坏了， 或者，随着你对笑的理解加深， 你就越来越擅长做这个， 你不再单纯因为听到别人笑 就想跟着笑了。