I do believe in negativity bias. It's a psychological tendency for humans to give more weight, attention, and memory to negative events, information, and experiences than to equally intense positive or neutral ones.
It has some evolutionary roots, where failing to notice a potential threat could be fatal, whereas missing a positive was merely disappointing. Paying attention to the negative helped keep us safe.
But now that we are secure, that same explanation no longer applies, and we no longer perceive negative experiences as survival instincts and more to do with protecting ourselves emotionally.
Maybe, at times, we are just looking for reasons. Maybe how we see another person is shaped by what we need to believe to make peace with our situation.
We love them, but can't have them. So perhaps we focus on the things that hurt us, like the moments that make their absence feel easier to accept. Slowly, those negative experiences begin to outweigh the positive ones
There could be a thousand good moments, yet a single painful one becomes the one we keep returning to.
Maybe not because it outweighs all the good, but because it helps us accept what we wish were different.