How does one distinguish an excuse from a reason?

This has always confused me. The two terms exist, reason and excuse, and one seems to describe why something could not have been as expected, while the other describes an attempt to deflect or lessen blame for the same instance. It seems clear to me which is which, however when I see given, or myself give, a reason for something it is always called an excuse. Is it that our society is so conditioned for self centered thinking that anything not being as “I” expect it to be cannot be justified, and thus there is no reason why it would be this way?