I have spoken about this in the past but it is worth repeating.
Before you criticize someone - remember: This person is going through a lot more than you know. What you say will make them feel bad not only about what you actually said but will also make them feel worse about the other things in life that they are suffering from.
It is also worthwhile remembering that you might forget what you said a minute later. The subject of your criticism, however, might walk around with the hurt for weeks. Or months. He/she will probably get over it eventually but might never forget it completely and every time he remembers, your words will sting him anew.
Also, remember that we are not Hashem's policemen. There is a mitzva of giving rebuke but there are very specific halachic parameters that must be adhered to. Otherwise. the rebuke is an aveira. הוכח תוכיח את עמיתך ולא תשא עליו חטא.
People care little about your accomplishments, talents, abilities, academic degrees and what you do with your life. People care primarily about how you make them feel. EVERYBODY wants to feel loved and accepted. So if you MUST say, don't forget to ALSO give the person lots of praise and appreciation for who they are and what they do. When someone only speaks up when they have something negative to say [which is very very often the case], it is hard to accept criticism from them. One of the conditions for giving rebuke is that the person will listen. So make sure they get lots of positive from you so that they listen when they get negative.
