(Plus their relationship started as them BOTH treating the other as investment/investor + a bit of manipulation from Mel. Not saying Mel is a bad person or anything, but I fear characters are a bit more complex than that and she was, thank God, not a perfect and always morally good character. The same way that Jayce definitely profited from Mel's knowledge on politics and 💸money💸)

They wanted to help the other achieve their dreams and their interests matched in season 1. They brought comfort to each other. Mel's affection for Jayce literally shows how much she cares about people and how truly vulnerable she is behind the cool, powerful and manipulative mask. In season 2, Jayce was focused on Viktor/the arcane pronlem and Mel on Ambessa/the litteral war problem. Their interests didn't match anymore (except for like... saving the world which brought them back together, ugh THAT SCENE WHERE HE TELLS HER SHE'LL NEVER BE A PASSENGER), and when they reach their final conversation we know by then that Mel will leave Piltover to go on a path of self discovery while Jayce is still focused on Viktor. Even if he had "survived", he would have stayed in Piltover to help rebuilt it or shit, idk.

Mel has to discover herself because she had tried to be someone else (the Medarda archetype) for years which didn't work out and now she has to find HERSELF. Jayce held resentment against her because not only did she really manipulated him (again, doing one wrong thing does not mean she's the worst person ever, plus she had reasons) but also a part of him holds her guilty of not saving everyone in the council (especially Viktor I'd assume since it's kinda his death that threw Jayce into the shit he went through in season 2). He BLAMES her. He's wrong about it, he feels like an asshole about it and he apologizes about it but he did have the thought. It's a common case of feeling guilty about a tragic event (because you survived) and redirecting the self blame toward someone else as soon as you can.

Once again them not being perfect is GOOD. Their relationship MATTERS to the story and their characters. He ADMIRES her, especially in season 1, where he imitates her in a way (also even without that Mel is literally the most beautiful character in Arcane come on.). She admires him too, not only his ingeniosity his heart and his want to help people.

IT WAS REAL TO ME.

IT WAS REAL TO ME.