Because we wonder what's wrong with us. What we did to make this person no longer want to be with us. We feel rejected. Unwanted. A lot of powerful emotions can follow a break up with someone who we loved and respected.
But we have to remember that there really is no right or wrong answer to breakups- in the vast majority of cases, it's just a matter of preference.
Think about it; what one partner loves will very often drive another person absolutely crazy.
For example, one person considers you distant and unavailable. But the next one finds that you give them the room they need to not feel trapped. Or your ex thought you didn't ever want to go out and were anti-social. But then your next partner loves nothing more than a quiet night at home watching movies with just you.
What one person saw as a negative worth breaking up for... another will see as an attribute that makes you ideal for them.
The real challenge in any relationship is simply finding someone who loves you for you.
So the "aha moment" in any breakup is realizing that in the vast majority of cases, there was nothing wrong with us, we just had a bad fit. And now our challenge is to take this knowledge, and find a better fit.
This is why it's always a good idea to be our unapologetic, authentic selves whenever we meet someone. People should proudly be who they are and let the chips fall where they may, as it's the primary requirement to finding true, lasting love.