An excerpt from WiseGeek.com:
Constructive criticism is criticism kindly meant that has a goal of improving some area of another’s person’s life or work. Often constructive criticism refers specifically to the critique of someone else’s written or artistic work, in perhaps a teacher/student setting, that would allow that person to further improve the work or to improve their approach to future endeavors. . . . Parents also try to employ constructive criticism to help their children improve their lives. The trouble with constructive criticism is that not all people are receptive to it. They may either feel their self-esteem shrinking under criticism, or they may feel that all criticism is negative. This can destroy the intent of constructive criticism.
Further, not all people who think they are employing constructive criticism are actually being helpful. They may think all criticism is helpful and may not spare the person any details or couch the criticism in ways least likely to make a person defensive. Communication is loaded with multiple intentions, especially in a parent/child or spousal relationships. Thus people may not know how to actually employ a critique of one aspect of a person without involving their own feelings or frustration that make a critique negative.