One way I heard it explained is this:
Imagine if your grandfather were a billionaire. But he was also a compulsive gambler. And he ends up spending his entire life savings on a poker game and loses. What’s going to happen to his descendants? They are going to be born into poverty. This might seem unfair, it is unfair, but it’s the way life works. The grandfather made a decision, and that decision affected all of his family.
Our actions have consequences. (something we tend to forget today in America but our ancestors were keenly aware of) Not just consequences for ourselves, but for the people around us as well. If a pregnant mother gets addicted to cocaine, her baby will be addicted. The baby didn’t do anything, but now he has to live with the consequences of his mothers poor choice. We are not isolated individuals, we are all connected.
So with Adam and Eve, they were created in perfect union with God. There were two benefits that this union gave them. 1. Eternal benefits. They had access to the Beatific vision. 2. Physical benefits. There was no sickness, death, or disease for them. Not because those things didn’t exist (they did in the animal kingdom, according to Aquinas) but because their union with God prevented those things from affecting them. But when they sinned, they cut off not just themselves, but the entire human race, from communion with God. As a result, humanity lost the gifts it had which were only available to it because it was united to God. Just like if a tree gets cut off from its root, it withers, we got cut off from our ‘‘root’’ - God - and so we fell back into the state that all nature falls into apart from God’s grace. Death, disease, suffering.
So that’s why there is original sin, and all the things that entails. It’s not so much that we are being ‘‘punished’’ for Adam’s sin. But we do have to deal with the consequences of it. Thankfully though, that’s not the end of the story. Christ’s death and resurrection undoes the sin of Adam. He heals and bridges the gap between God and man. He infuses sanctifying grace - the grace that Adam and Eve had but lost - back into our souls and we have access to heaven again, as long as we are united to Christ. And in the resurrection, our physical bodies will be completely made new, to something even better than Adam and Eve had.
Does that explain things a little better? Let me know if something still doesn’t make sense. And sorry for such a long post. I wasn’t planning on writing that much when I started haha.