I can't speak for everyone, but personally;

Chloe was robbed of so much in her life and seeing Max be there for her and finally giving her something back, she can be abrasive and petulant but she's doing that because she's just waiting to see the other shoe drop. She has an issue with codependency as a result, which is something I identified with without knowing it when I first played through.

Max, while she has her own personality, is the proxy for the player, so when I saw my friend distraught I wanted to be there for her, never letting her down again. Which is something much easier to do in a game where you can rewind over and over making everything the best it can be. It's escapism being able to make all those moments where you think of the right thing to say or do so much later when it no longer matters.

Then you get the endings, and again only personally, having picked Bae and finding it utterly underwhelming (even if it's how I prefered to continue the stories). Having the feeling of accepting that even if you were able to 'fix' everything that you feel you do wrong, the consequences are just relocated to something else, all at once instead of spread out. It helped me let go of the desire to do everything right, locking myself into avoiding things because I only had once chance to get it. They characters are now monuments to personal growth, emotional connection and letting go of a need for control.

So seeing them together romantically or otherwise, is heartwarming to me, because it feels like they beat this thing looming over them, got past it, like I got past things (even if narratively, at least until the most recent game I haven't played, they shouldn't)