The quickest was to lose a friend? Tell them what's wrong with them.
This holds true for any relationship - friends, family, colleagues or acquaintances. When you point out their faults, you lose status with them. They may not say it, but they're thinking it. Know that many are easily triggered and don't handle criticism well. This becomes even more important after 50. Why? Because who we are (faults and all) become harder to change, and many of us resolve to accept ourselves (and wish others would as well).
Instead, we prefer to surround ourselves with people who make us feel good about ourselves.
The more we love others unconditionally (faults and all), the more we'll receive the unconditional love from others. Dogs have learned this, why haven't humans?
Who does criticism hurt the most? The one being critical.
Every time you criticize someone, you criticize yourself (for being critical of another). Likewise, anytime you compliment someone, you compliment yourself (for giving the compliment). Strive for a 10:1 ration. For 1 criticism, you give 10 compliments (to yourself or others).
Don't be critical, critique instead! Only point out faults if you can offer suggestions. Unless you can offer suggestions, you have no place pointing out faults.
But don't beat yourself up for being critical either. The best way to stop the loop is to inject a compliment.
All the kind words you say will be forgotten when you say our send words of hate. I have experienced this myself and while I wish I could take back the words I said, I never can. But what I can do is note the bug in my program and use it to run a less buggy program today.
Which words hurt the most? The ones we speak to ourselves. Hurtful words we say to ourself are the precursor of hurtful words we say to others. It's a form of priming.
To be authentic is to leave ourselves exposed to the opinions of others. Just remember that no two opinions will ever be the same. For every opinion, the opposite is true. If the opposite can be true, than neither opinion is. Opinions are made up - they are not true. They are simply the delusion of another. Opinions from others will only hurt you if you allow them to.