There are many different parts of everyday life, and there are many different areas in which people can be mature or immature. If you are immature all of the time, nobody will ever take you seriously, and you will usually be a big pain to have around. On the other hand, being mature all of the time would mean having next to no fun. What people need to learn to do, in order to have enjoyable interactions with each other, is learn what to be mature about, and what not to be mature about.
Often, when somebody bothers you, this is why. They are being immature and silly or annoying when you're in a serious mood, or having a serious conversation. Or, they might not know how to relax and stop caring about things. There are people at both ends of the spectrum, and neither are particularly enjoyable after awhile.
Naturally, there are certain social circumstances in which it is vital to be mature (such as a funeral) and we look down upon those who are not. However, once the situation passes, we expect everybody to be relaxed again. Some people seem to not know how. The same goes for immaturity, and we as humans expect everybody to know what comes when and to strike a healthy balance between the two. When people carry a level of maturity into a place we don't expect or want it to be, we treat them with the same indignation as we would an actor who stays in character even when not performing. That is, with some frustration.