There's a study out there that says
happiness is contagious. Not just from/to your nearest and dearest though - people more removed from you have a greater impact on your mood. There's some skepticism in the article about that - the idea that your neighbor can affect you more than your spouse - but I believe it. We get so comfortable with the people we see all the time that we've probably gotten used to their fluctuations in happiness and the like.
As someone that has been having a truly wretched week, I think I need some extremely happy people in the upstairs apartment.