First, competition often makes kids anxious and that interferes with concentration. Second, competition doesn't permit them to share their talents and resources as cooperation does, so they can't learn from one another. Finally, trying to be Number One distracts them from what they're supposed to be learning. Just because forcing children to try to outdo one another is counterproductive doesn't mean they can't keep track of how they're doing. But any mother who values intellectual development for her child should realize that turning learning into a race simply doesn't work.
Competition is a recipe for hostility.
By definition, not everyone can win a contest. If one child wins, another cannot. This means that each child inevitably comes to regard others as obstacles to his or her own success. This is not to say that competitors will always detest each other. Existing relationships are strained to the breaking point, while new friendships are often nipped in the bud.