Reasons vs. Excuses

rea·son\ˈrē-zən\
noun
: a statement or fact that explains why something is the way it is, why someone does, thinks, or says something, or why someone behaves a certain way
: a fact, condition, or situation that makes it proper or appropriate to do something, feel something, etc.
: the power of the mind to think and understand in a logical way

ex·cuse\ik-ˈskyüz,
1 a : to make apology for
b : to try to remove blame from
2 : to forgive entirely or disregard as of trivial import : regard as excusable <graciously excused his tardiness>
3 a : to grant exemption or release to <was excused from jury duty>
b : to allow to leave <excused the class>
4 : to serve as excuse for : justify <nothing can excuse such neglect>
Other forms: ex·cused; ex·cus·ing
ex·cus·able \ik-ˈskyü-zə-bəl\ adjective
ex·cus·able·ness noun
ex·cus·ably \-blē\ adverb
ex·cus·er noun
Origin: Middle English, from Anglo-French escuser, excuser, from Latin excusare, from ex- + causa cause, explanation.
First use: 13th century
Synonyms: blink (at), brush (aside or off), condone, discount, disregard, forgive, gloss (over), gloze (over), ignore, overlook, overpass, paper over, pardon, pass over, remit, shrug off
noun
: a reason that you give to explain a mistake, bad behavior, etc.
: reasons that you give to explain politely why you cannot do something, why you have to leave, etc.
: something (such as a condition or set of conditions) that explains improper behavior and makes it acceptable
Full Definition
1 : the act of excusing
2 a : something offered as justification or as grounds for being excused
b plural : an expression of regret for failure to do something
c : a note of explanation of an absence
3 : justification, reason
synonyms see apology
Examples
What’s your excuse for being so late?
She had no valid excuse for not finishing her homework.
He’s always making excuses for himself.
First use: 14th century
Synonyms: alibi, apology, defense, justification, plea, reason

At what point is the line drawn between listing reasons and listing excuses? What determines the difference?