I thought it might require a lengthy essay, but I think I can actually condense it to a paragraph or two.
Why do we pay so much more attention to pain than we do to joy? Why are we so much more affected by failure than by success? I'm probably generalizing; introverts like myself are more likely to pay excessive attention to either, and I've seen such people leaning quite hard to the side of pain and stress and hardship.
The easy answer is that pain is uncomfortable, so we notice it more and are more motivated to learn how to avoid it in the future. In other words, pain and stress are more likely to produce changes in how we view and interact with the world.
I find that almost unbearably sad. That the truly good things, love and joy and peace and pleasure and beauty can so easily be relegated to the default position of "things that are not pain."
I wish pleasure and joy produced the same kind of changes in us that pain and pressure do.