Hot take: Nathan is a narcissist and not a good guy!
Often, I see Nathan described and sweet and sensitive, but I feel like he's a kind of a bad dude!
His relationship with Skylar shows a pretty recognizable pattern of love-bombing, validation-seeking, selfishness, and making his insecurities someone else's problem.
True to narc form, the aggressively lovebombed Skylar and needed constant validation. He publicly cheats (with very little remorse) the first time he gets attention from someone new, instantly replacing his boyfriend with Zach. After realizing Zach played him, he runs back to his old supply (Skylar) and expects him to take him back. Nathan's biggest issue isn't simply cheating. It's the contrast between how intensely he pursues someone when he wants their love and how carelessly he treats them once he has it.
Much like a narcissist, he seems less addicted to a particular person than to the feeling of being wanted... I feel like Skylar gets a lot of hate online, but it's Nathan that's the real villain in this story!