Constructive criticism is "speaking the truth in love," as God's Word calls it. It says in Proverbs that a blow from a friend is a good and welcome thing. True friendships bear the responsibility of striking the other when necessary. It is not an option. A person who can never tell you the truth, who can never confront you in love, is not really your friend. You may hang out a lot. You may like the same music. You may laugh together. But if there is no honest communication, even about negative things, then it is not a friendship.
Constructive criticism, as implied above, is built on a foundation of love. There is already an established relationship of trust and acceptance and safety between the two people. This ensures the other person, "Listen, I'm not throwing you away in the garbage. I still love you. However, you need to look at this." It ensures them that you are not there to reject them, nor are you there to slay them with your words for your own benefit or exaltation. You actually care about them, and there is no question about it in their minds.
As Paul tells us in 1 Cor 13, love does not seek its own. Therefore, constructive criticism is not just a "nice" way to point out the flaws of another person for your own selfish benefit. As stated, it is built already upon acceptance of the other person. This is diametrically opposed, then, to a relationship that is built upon performance. Yes, sadly, those exist. "If you do this and if you live up to this then I will love and accept you. If you do not, I will condemn you, reject you, and eventually abandon you."
Constructive criticism is redemptive. It is hope-oriented. It looks forward to a better picture and comes alongside the person.
What constructive criticism is not:
You'll always meet someone who is ultra-critical of you and anybody they get close to. They justify their behavior, often, by saying things like, "Well, its true." I've actually had someone, ironically a victim of various kinds of abuse themselves, tell me that their behavior toward me was not wrong because their words were "true." Even if it is true (though more than likely a partial-truth spun through their selfish perception), this kind of person fails or refuses to realize that the "truth" can be used as a destructive weapon.
There is no love in these words, no matter how nicely and calmly they are spoken. These words are intended to hurt. Regardless of the degree of truth to the words, using our words as a weapon to punish, demean, silence, belittle, dominate, control, or manipulate someone is abusive.
When someone compares you to others as a way to point out your flaws or how they are unsatisfied with your performance, that is abusive. There is no foundation of acceptance. They are seeking to punish you for how you have dared not satisfy their cravings, or they are hoping if they crush you, you will "step up" to the plate and try harder. They are not loving you like a person but using you like a tool to get what they want or, in some cases, stepping over you like an obstacle. You are being used as a means to an end.
Next time a person speaks to you in such a manner, ask them, "Are you looking out for my betterment or the betterment of the relationship, or are you simply pissed off at me for not giving you what you want?" And then watch them deny, deflect, and blame-shift. That is what abusive, critical people love to do. They won't face the music for how they treat people.
Abusive criticism destroys hope. There is no redemption in it. It tells you, "You're no good. Nobody would ever want you." It condemns, end of story. It is meant to give you a little piece of hell on earth -to destroy, to put you beneath them, to punish and discard you for failing to meet their selfish cravings. It does not seek to wound in order to root our the bad and make room for more good. It is not like pruning a tree. It is more like poisoning the root and smashing the stump. It is endemnic.
Abusive criticism, finally, isolates the victim. Nobody comes alongside you to hope for your best. It is intended to put down, to shame, isolate, and to make you feel separate from the rest of humanity or a certain group of it. It is intended to segregate you from the "worthy" and place you in the "unworthy" class. It does not seek your good. It seeks your slavery and destruction. It wants you to buy into the idea that the abuser is right and that you need to somehow fix yourself of you will be exiled to a worthless oblivion forever. Nobody will want you otherwise.
I've been on the receiving end of this kind of thing. Let me tell you... words do hurt. Words do wound, far deeper than physical blows. Condemning, demeaning, hope-sapping, dignity-crushing words, even if you believe they are "the truth", do lasting damage to people. Words have power. Maybe you can break a person's arm with your hands, but you can crush a person's spirit with your tongue. A broken arm will heal in a few months. A broken spirit may never heal fully.
Be careful with your words.