Obviously everyone can use some improvement in what they do, even the greatest artists of all time seek to further enhance their skills.
That aside, constructive criticism is a lot different than simply telling someone they can do better and pointing out their flaws. Take a minute and think of your first piece of art, you thought it was great, you knew it could be improved upon with time and effort but you were proud of yourself nonetheless and desired to show someone what you could do.
A little bit of positive reinforcement goes a long way, if you are great at something you should seek to help others while also complimenting them and taking a look at their work as it is. Art is unique and no one can tell another that their art is wrong. A suggestion on how someone can improve their art is only worth something if the artist being critiqued see's any value in the response. Lend a hand, not a blade.