Because we are fallen as a result of the sin of our First Parents.
As I was being introduced to what Catholicism taught and expected of its adherents, I realized that there was a range of behaviours we call sins that I was going to have to divest myself of if I wanted to make a go of this thing. And that pattern would bring me to the point of asking an inevitable question, often. If sins are wrong and God made us and the universe according to a certain intelligibility whereby his mind and intention is revealed to us through nature, why did he make some sins feel so good? In order to answer that question, it might be helpful to look at a micro use case that so exaggerates the qualities of what we’re talking about here, that it might make it easier to understand. From what I’m told, if your greatest ambition in life is hedonistic pleasure, then crystal meth might have some appeal for you. According to some page, I found on the internet, this is what it’s like to consume meth. The first effect is the “Rush” in which the user’s heartbeat, metabolism, and blood pressure take off surging the drug and the effects throughout your body. I’m not sure how accurate this is, but I’ve been told that meth raises your dopamine levels, which is a pleasure chemical in your brain, to levels as high as 7 times what you experience with sex. Next, the high kicks in and this is where you experience an unrivaled euphoria that convinces you that you are the most important person in the world. Next, you experience the binge in which you become so obsessed with the high you’re experiencing that you start to do whatever you can to keep it going, usually translating into taking more meth. This can last as long as 3 to 15 days. With each boost of the drug, the user experiences another rush that lessens each time until there is no more rush or high. Then the user experiences tweaking when it is impossible to continue to feel the high and the catastrophic effects on the brain become pronounced. The user starts to experience horrible feelings and craving that can’t be satisfied and at this point, they’re in a completely psychotic state trapped in their own nightmare of hallucinations and psychosis. And maybe you’ve been in a situation, where you’ve done something you believed to be a sin and asked yourself or God if this is so bad, why does it feel so good. I think there are a couple considerations that can bring some insight to this question. The first is that most sins are not some immersion into utter depravity, but a perversion of some good. Like sex is a good that God created as a part of life, but we can abuse sex through perversion, but those perversions still retain some measure of the good that God created it with, so there can still be experiences of pleasure even though the act has been compromised. The second thing is that while sin may be bad, while you and I still retain some measure of goodness and innocence, we are capable of experiencing the pleasure of what it means to be good. So with the meth example, a person who has never done meth or other drugs which alter their brain chemistry is someone who can experience the pleasure the drug promises because their brain is healthy. But what meth does is it takes what is good in that person, their normal brain function, and abuses its potential by releasing an extreme surge of dopamine that causes an experience that the brain would not normally produce. So, because they have a healthy brain, they are able to experience the pleasure that meth promises, but as the example reveals, after using the drug, their brain succumbs to a state of affliction from which the drug can no longer produce the pleasurable effect. And this is how all drugs and all addictions work. The first time you do it, you experience a pleasure that convinces you that this same trigger can produce the same pleasure any time you want, but as addiction sets in, that same experience of pleasure moves further and further out of reach, until you’re just using to fend of the withdrawal. That’s a profound allegory or even demonstration for the nature of any sin. Some sins, when you first embrace them, can produce a pleasurable experience, but that’s not because the sin is good, but because you are good and that which is good can experience what is good (that is a pleasure), even if it’s tainted. But each time you embrace some compromise of sin, it changes you from the person who was good and, therefore capable of experiencing the pleasure of the experience, to someone who is intrinsically less good. CS Lewis puts it this way. “Every time you make a choice you are turning the central part of you, into something a little different than it was before. And taking your life as a whole, with all your innumerable choices, all your life long you are slowly turning this central thing into a heavenly creature or a hellish creature.”