The most common reason children become aggressive, though, is because they've witnessed aggression. If your child has been exposed to violence, whether at home or in a place where you have less control over what happens, take steps immediately both to ensure it doesn't continue and to help him understand that it shouldn't have occurred. If your child's aggressive behavior is frequent and severe, or your efforts to curb it have no effect, you'll need to consult your pediatrician or a trained mental health professional, such as a child psychologist or psychiatrist. All preschoolers occasionally grab a toy from a classmate or scream themselves into a full-out temper tantrum. An aggressive child will act this way in more than one arena, such as home, school, and social events or athletic activities.
Physical fear is one straightforward explanation.
Your child may lash out, for example, if he feels cornered by another child. Other reasons have less to do with instinct and more to do with circumstances. These are years when children are learning a vast array of new skills, so they can easily become frustrated. Being asked to adjust to new routines, such as daycare or preschool, can also mean less time at home with their parents. If they feel resentful or neglected, they may react by pouncing on the nearest playmate.