A - Thanks for your question. The reason we sin is because we have authentic freedom. The story of Adam and Eve tells us the story of humanity. We were created by God to live in relationship with Him. But, he did not want us to be mere puppets or robots, but to love Him back. To be able to love others means that we must be free to choose to do so, because true love is choosing what is best for another despite the cost to oneself. On the flip side, this means we are truly free to not choose to love another. This is sin, when we choose not to love. This is what happened when Adam and Eve chose their own will above God's will and we all suffer a fallen nature because of it. This leads to what the Church calls concupiscence - that is, the tendency to sin.
Another way of phrasing this question would be - "why do we love"? Because it is the other part of freedom. We are humans and we all sin. But, we can all love as well. This is what we created for - love. To live in an eternal relationship of love with God and His creation. But, we get the choice - love or sin.
One way of understanding sin is to understand it as seeking a disordered good.
Some may ask - what about Satan? Well, Satan does exist and does tempt us. But, we still must choose the evil for it to be a sin. Temptation can come from our own desires, from the temptation of the world, or the temptation of demons. Regardless, we still have a choice.