Willpower is limited.
If willpower is limited, then desire erodes willpower over time. If 2 people have desires for one another, then it shows, and both of them tend to undermine the other's willpower.
If you are in the presence of temptation, and don't get your (uncontrollABLE) desires in line, then temptation always wins.
If Neitzche is right:
"I have done that," says my memory. "I cannot have done that" -- says my pride, and remains adamant. At last -- memory yields.
then one can imagine how well "should" resists "want".
Remove temptation, or succumb.