Great question, I think that reason and excuse suggest meanings (Maybe only to me) that are 180 degrees out compared to the social connotations attached to those words.
What the words suggest to me:
excuse - a reason that excuses your action
reason - reasoning that led you to believe that you could do something but does not excuse you of guilt
What society seems to think of the words:
excuse - an unreasonable fabrication to cover up ones guilt that does NOT excuse the person of guilt!
reason - A good explanation that provides an excuse
What the words suggest to me:
excuse - a reason that excuses your action
reason - reasoning that led you to believe that you could do something but does not excuse you of guilt
What society seems to think of the words:
excuse - an unreasonable fabrication to cover up ones guilt that does NOT excuse the person of guilt!
reason - A good explanation that provides an excuse