Or the causation arrow could be the opposite, repeated rejection or lack of relationships ultimately causing issues too, including mental health ones. I've see

Or the causation arrow could be the opposite, repeated rejection or lack of relationships ultimately causing issues too, including mental health ones. I've seen papers on both sides. Unfortunately we don't know which hypothesis is correct just yet. Previously, rejection sensitivity was not a problem due to institution of arranged marriages. Nowadays, you do not even have proper meeting places for shyer or more introverted people, or even properly practice social relationships at times. We'd need an anthropologist to check the structure of parties over time and cultures against rates of remaining single...