Article:Taming Toddler Aggression

If you fear that your toddler will never have any friends because she won't share her toys, take heart. Conflicts over sharing usually subside by the age of 4, when kids not only understand better why sharing is important but also have greater self-control.

Another less-than-pleasant by-product of "toddler togetherness" is aggressive behavior-hitting, shoving, and even biting. These unpleasant bullying tactics often stem from anger or territorial issues, the same egocentric mentality mentioned previously. But other instances seem almost random (and, hence, can be highly worrisome to Mom and Dad).

http://www.parents.com/toddlers-preschoolers/discipline/improper-behavior/toddler-aggression/