I recall someone once told me that it is because the hearts were not in one accord.
This is just an observed interpretation, not an explanation. I believe it is due to hypocrisy, over commendations, pretend (incredibly well trained to a point even one fools himself) to act nice, etc.
Everytime when someone comes up to me and say: "Tim, you play piano so well, play violin so marvelously or sing in choir so wonderfully", I would often think to myself, "Am I the only one here who seeks perfection?".
We are to serve with each other in the HOLY SPIRIT and in TRUTH. Of course, as for harsh truth, one requires the wisdom of discerning who is serving with the fear of God and therefore able to take criticisms maturely.