You’re not forced to become a doctor. But if you do, and you do so not in isolation but with the resources provided for the state, resources that include efforts and labour by the proletariat of the state, then you’re expected to give back to those that will come after.

I truly don’t understand how that concept is so difficult to understand.

Debt to society is a figure of speech, not a material debt. It’s about participating in society that enables your life. If you don’t want to, you can always move to the woods and live in a cabin away from civilization and do everything yourself. But if you want the benefits of society and civilization, then you have to realize that you need to give back as well. It’s literally not that difficult a concept to understand, I’m not sure what’s confusing you