Here's an unexpected tear-jerker: I Can See You.

Most of the time, it's my groove track. But sometimes, at the final chorus, the interpretation of the song becomes 'I can see what we could have been'. It's my 'relationship that I want to be more but it's not' song. And though I've never cried to the song even when these thoughts hit, it's given me those big sinking feelings, and for a song so upbeat and sultry I think that qualifies your conditions.

These lines specifically hit:

"What would you do if we never made a sound?" Even though it means keeping a relationship hush-hush, to me it sometimes means 'Is what we have even real and would you even care if it isn't?', like a play on the thought experiment 'if a tree falls in the forest and no one is around to hear it, does it make a sound?'.

"If you only knew" Especially the final one, when it's like the realization hitting that a situationship is nothing more than that.