Curate, connect, and discover
reading this while listening to are you satisfied by marina and the diamond LORD
It's certainly interesting to think about the fact that Gi-Hun and Sang-Woo envied one another.
While it might be obvious to see why Gi-Hun would envy Sang-Woo given his credentials; Pride and joy of Ssangmun-dong, top of his class, SNU buisness graduate, valedictorian, etc. It might not be as obvious to think what Sang-Woo envy Gi-Hun for.
I think the answers relies on what society considers a "successful man"
This is usually measured by these factors: finances, career and family.
While certainly on a superficial level, Sang-Woo reach far greater than Gi-Hun in terms of career and finances, Gi-Hun had a title that Sang-Woo didn't.
Husband and father.
They shared the experience of both having success in their careers and having that taken away from them, and their finances were fucked over because of it.
But, unlike Sang-Woo, Gi-Hun was a "complete" man. Even though they both "failed," Gi-Hun had a daughter that would thrive regardless of the bad decisions he made. His legacy would live regardless if he had a career or not.
Sang-Woo only had himself and his achievements, and he lost everything.
Even though he seems quite disinterested with the idea of having a girlfriend (more than likely he is a closeted gay man) it's the fact that Gi-Hun build something everlasting, even if it didn't require any special titles. Just the will and the love.
If instead of reaching for something beyond his league as it is to become a multimillionaire, he focused on the "small and trivial" things, he might have built something of importance, something that wouldn't die along with him.
He envy the fact that Gi-Hun, with the contrasting careless attitude, could build that for himself. Could be materially more successful than he is.