Why do I need to change my nature?
Some people wonder why Hashem would give us a nature to lust, and yet ask us to change our very nature and reign it in. "How does Hashem expect me not to go after the nature that He Himself gave me?"...
It's interesting to note that it is brought down in Chazal and in the Zohar many times how Shmiras Habris is part and parcel of the mitzva of Milah that Hashem gave for every newborn. The mitzva of Milah is the sign or "stamp" of Hakadosh Baruch Hu that becomes part of our body at 8 days old, and this mitzvah continues throughout our whole life as we "guard" that stamp of Hashem from entering into "the other side" by using it in sin. In a number of places, the Zohar equates the mitzva of Milah and Shmiras Habris as one and the same, and when Chazal say that Avraham Avinu pulls out every Yid who has the milah from gehinom, the Zohar says it only means one who has fulfilled the mitzva of milah properly by guarding it during his lifetime.
In the same way that our bodies are given to us by Hashem with a "blemish" that needs to be removed (the orlah), so too, we were given a nature to lust by Hashem, but this "orlas ha'lev" needs to be removed. For many of us, removing the lust from our hearts can be as painful as the bris milah we had at 8 days old. But in the same way that we defy nature by removing the orlah as a baby, so too we are asked by Hashem to defy our nature on an even higher level as we mature, and remove the orlas ha'lev of lust from our hearts.
A Roman officer once challenged one of the Chachamim about why we Jews make Milah; "Why would Hashem make the body imperfect?" The Tana returned the next day with sheaves of wheat in one hand, and freshly baked rolls in another hand and ask the officer which one he thought was more praiseworthy. In other words, Hashem gave us an imperfect body so that we should perfect it! In the same way - and even more so, Hashem wants us to perfect our hearts and nature. That is our job in this world, and for that we are rewarded more than any one can imagine!
In continuation of this idea, I'd like to bring an excerpt from Rabbi Avigdor Miller - Tape 933. (Sent by simchashachaim.com)
In Gehinom, the treatments are not as easy as the treatments in this world. In this world you can take a Sefer (book) like Reishis Chachma, or Shevet Mussar, or Peleh Yoeitz, or Orchos Tzaddikim, or Mesilas Yesharim--these books will be a salve, a balm on your blister of envy and they'll cause it gradually to go away almost painlessly.
By learning the right kind of Torah a person can change his character and can remedy his faults almost painlessly in this world. But if he neglected to do that and he goes to Gehinom with that blister, with that ulcer, there the treatment is entirely different.
There no more Mesilas Yesharim, no more Orchos Tzaddikim, all these good remedies are left behind in this world. And there they have very powerful chemicals, like burning lye, they'll put burning lye on that ulcer and they'll remove it. But it hurts like Gehinom. And this I cannot describe to you because it's indescribable.
A person who has even a single bad Middah (quality) is going to undergo a terrible torture. So if he's able to rid himself in this world of it, he's a wise man. But if he waits, Hashem in His mercy is going to heal him but he'll have to undergo the treatment.
Don't ask me why should that be so. You want the next world with all privileges? Hashem gives certain privileges to people who are smart enough to make use of their lives, and people who are stupid enough to live with bad character and they wait for the next world, they'll just have to take it despite all the questions.
Don't say Hashem, since You are kindly, couldn't You have treated us without this pain? He'll say to you, Why didn't you do it when you had the opportunity? You want something for nothing?