Tearing in Anger
We learned recently in the daf yomi (Shabbos 105b - see below) that if someone tears a garment on Shabbos in anger he is Patur because he is not doing a constructive act but only an act of destruction. So at first the Gemara asks that it is somewhat constructive, since it helps to calm his anger? Responds the Gemara that it's forbidden to do and when a person listens to his Yetzer hara today for something small, the next day the Yetzer Hara gets him to do something bigger, and so on, until he ends up getting the person to worship Avodah Zara!
So Rashi addresses an obvious question. How does this answer the Gemara's question? Even though it's forbidden, it still serves the purpose of calming his anger! Answers Rashi, that by giving in to his anger he is just teaching and training his Yetzer Hara to come back again even stronger next time. Therefore, even though it seems to serve the purpose of calming his anger now, it is really just making his situation worse overall and is therefore considered destructive by nature.
This is a powerful lesson for us regarding the Yetzer Hara. Sometimes when the desires are strong and we feel like we can’t serve Hashem and move on with our lives, we think maybe it's worthwhile to "give-in" to the Yetzer Hara just to calm him down. But Rashi is telling us here WHAT A LIE THIS IS. Every single time we give in, we just train the Yetzer Hara to come back next time even faster and stronger than before!