Well, so far as I can tell, self-deprivation of hedonistic pleasures is necessary to lead a good life, according to most religions. Discipline of the self is a trend ranging across all religions. But religion isn't the only thing that believes excessive hedonistic pleasure to be self-destructive. Ancient Greek philosophical schools such as the Epicureans and Stoics also believed that discipline of the self was the only way to find contentment in this world. The "big three" Socrates, Plato, and Aristotle all believed virtue and discipline of the self was its own reward. So I don't see how avoiding "sins" according to religions is itself a bad thing. If anything, religions ought to make people more self-restrained with their lives.
Secondly, eternal torture varies from religion to religion. Some people believe what is called annihilationism, which is the belief that God destroys the soul after death, as opposed to eternal torture. So not all religions hold to that belief, it would seem. (Although I do see the point you were driving at)
Now, speaking from a purely Christian perspective, the reason I (and those Christians that would agree with me on this) restrain myself is because I recognize the fact that I am a creature made in the image of God. As such, I desire to live a life that mirrors His nature. The whole reason He created me was to bring Him glory. So, I do so by mirroring His nature, which is what we call good.
As for sin, the reason I do not wish to sin is so that I can bring all due glory to my God, and not any form of reproach. You see, if I am an ambassador for a nation, yet I do not reflect my nation's character, beliefs, and teachings, then what sort of ambassador am I? I am no ambassador at all, I am a slanderer of my nation. It is similar with God the Father. Since God has made us in His image, He expects us to act like Him. When we do not, we sin, and essentially bring dishonor to His eternally-worthy name.
What, then, is God to do with a man that represents Him and His glory, yet misrepresents Him and only brings Him shame? Obviously, the answer is to remove us from office, so to speak. What precisely that entails is debatable, and we will never really know. But with knowing God as eternally and infinitely worthy of all glory, even what we would consider a small offense is an infinitely grave reproach against His name. And it stands to reason that, if the punishment will be just, a steep price will be paid.