There are a number of reasons why your toddler shows aggression at this age; many toddlers do:



A drive for independence and identity:

As your toddler's world expands to include playground, playgroups or nurseries, he generally asserts himself in order to feel larger and more important.



Frustration:

Your little control freak gets upset when he is unable to control his environment and reacts the only way he knows how; by biting, by slugging or by pushing the other child who gets in his way.



Egocentricity:

Most toddlers halfway through the second year, still see themselves as the center of your world and display an inability to care for others. Many still view their peers as objects to be mishandled or discarded rather than as equals.



Lack of impulse control:

Even once a toddler understands that hitting hurts, he may not be able to stop himself from doing it. A toddler has the same impulses as an adult when he believes someone is giving him a hard time, but he has not yet learned to control them and hence modulate his behavior.



Inability to foresee consequences:

He may be sorry after hurting someone but he doesn't have the foresight to avoid the unpleasant result of not hitting his friend in the first place.



Lack of social graces:

Social skills are not inborn; they must be learned through experience, by emulating older role models, by trial and error and through the repeated instruction of others.



Lack of verbal skills:

Toddlers' actions speak louder than words; they don't yet possess the verbal proficiency required to express feelings, needs or desires. So most often they resort to physical means of expression.
Normal toddler aggression can be aggravated by external forces. Through elimination or even by minimizing these factors, you can help your child reduce his aggressive behavior:



Lack of sleep:

When there is a change in the nap periods and he hasn't adjusted to it, or he has been waking up in the night a lot due to teething problems, or he simply had trouble settling down the previous night, your child will suffer from lack of sleep and misbehave.



Hunger:

Going for long, long stretches between meals and possibly consuming foods high in sugar can trigger misbehavior.



Illness:

A suddenly aggressive child may be coming down with a viral attack or another kind bug, or may be just over one.



An unsettling life change:



A new nanny or parents taking a holiday without the toddler can make him more edgy than usual.



Inadequate attention:

Children often act up in order to win notice or approval for good behavior



An overly hostile environment:

Children tend to copy the violent behavior of their caretakers- parents, caregivers or sibling and this can fuel aggression in the toddler.



An overly controlling environment:

When the toddler is never allowed to make choices, his frustrations builds and with that his aggressive tendencies.



An uncontrolled environment:

Parents who don't set limits are unwittingly encouraging aggressive toddlers.



Dysfunctional environment:

When those around the toddler are immersed in depression or marital problems, abuse alcohol or drugs that interfere with normal functioning and parenting, the child may display his distress by acting up.