I only trust people who are willing to criticise me.
People, especially Christians, are too often too nice. If people only ever say nice things to me, I have no reason to believe that they mean them.
If people sometimes say nice things to me and sometimes say hard things to me, then I have more reason to believe that the nice things are actually meant.
Of course, there are some people who are just too negative, and some people are wrong. I don't trust everyone who criticises me, just the people who get it right.
It's kind of the corollary from that famous verse in Proverbs:
The wounds from a friend can be trusted, but an enemy multiplies kisses.
Proverbs 27:6, NIV
I count people as close friends when I can trust wounds from them.
Of course, the corollary to that is that I should be a good friend by giving trustworthy wounds, which I'm not always very good at. I need to be the change I want to see in others.
The people who only ever say nice things, comfortable things, to other people are actually enemies, because they are in it entirely for what they get out of it. Real friends give wounds.