Why do some people attend weddings and barely dance? Even the evil Queen Izevel danced in front of the Kallah and was rewarded that her hands and leg were buried.
When you dance for someone else you are forgetting your own problems, troubles, issues and worries. You are FORGETTING yourself and immersing yourself in the holy task of bringing joy to another human being.
It really bothers me to see people who walk in to simchas, say a quick mazel and make their escape. When we have our own simchas we have PLEEENTY of time - so why can't we find more than a tiny tiny fraction of that time for a friend. Is everybody SUCH A MASMID that he has to run back to the Beis Medrash. Or people who are running home - what are they doing at home that is so important??
Of course people need to get up early for work and have have other responsibilities.
But PLEASE - try to feel your friends simcha and show him/her that their simcha is yours as well.