We all know or remember that situation...
Someone told us something - and we were offended.
Outwardly, we may have "let it go", but deep inside - we were offended.
It may have been constructive criticism, maybe an intended insult - and maybe they weren't even talking to us.
But we took it personally, and were offended.
And then someone (else) who's opinion we value told us: "What do you care what they say?"
And I want to ask - and propose an answer to - the following question:
Why do we care? why can't we simply ignore any criticism?
The Sages speak positively and with high regard about people "...who are insulted and do not insult" - and I ask why should such people be valued?
Wouldn't it be better to credit those who totally ignore insults altogether?
Well, I'd like to offer the following (unfounded) answer:
The fact we care about what people say to us - is an important developmental mechanism.
The mere fact we care what people say to us/about us is a built-in mechanism that drives us to continuously try to improve ourselves - to become better than we were.
Because even if the criticism/feedback/insult is unjustified, the can motivate us change for the better.
And at the end-of-the-day - that is (or should be) one of our most important goals: to be better today than we were yesterday.
And tomorrow - better than we were today.
To always be moving forward - and never be standing still.
Good luck!
TL;DR;
I theorize we were created/evolved with a built-in self-improvement mechanism, which is fueled by input (feedback/criticism/insults)