Broadening the group beyond Catholic categories of mortal and venial, omission and commission that fulfill the prerequisites of 1) being wrong 2) knowing it is wrong and 3) being free and willing to do it anyway, let's condense the discussion to: why do people intentionally violate their own moral standards of right and wrong?
Mike suggested pride; Dan said greed. I offered low self-esteem. Another not-to-be dismissed reason: because it's fun. We are all right. There are probably as many reasons as there are people. A proud person refuses to admit wrong and continues in his or her offense until discovered or stopped. A greedy person always wants more. Why? ("Because he fears death." "Yes, Yes! Thank you for answering my question!" —Rose Castorini, Moonstruck)
But in men like Tiger Woods, incredibly talented, rich and famous, with a beautiful wife, why cheat?
1) Because he can
2) Because he feels entitled
3) Because it is risky
4) Because he thinks he can get away with it
5) Because he feels deprived
6) Because he feels depressed
7) Because he feels bored
We "holy" people sin out of a sense of compensation: "After all I've sacrificed, what's this little indulgence?"
We "institutionalized" people sin to manifest our independence: "I am not just a cog in this machine. See? Look what mischief I can do!"
Guilty people may sin to provoke God's wrath and unmask God's illusion of mercy.
Sad people may sin to escape their unhappiness. Lonely people may sin to ease their pain.
As we enter more deeply into Advent and do a little spiritual house-cleaning, instead of just confessing what we do, it may be beneficial to consider why we do it. Then, instead of focusing on how nasty we are, we might progress further in religious life by concentrating on how good God is.