There are certain types of anxiety that children experience. For instance there is separation anxiety. This usually peaks at about eighteen months, but can resurface on and off until well beyond kindergarten. Things like changing schools, getting a new babysitter, or even staying over at a friend’s house can trigger this type of anxiety.
Your child is also beginning to understand the idea of time and distance, and realizes that when you are away from him, you haven’t simply disappeared, but are doing something without him.
Then there is stranger suspicion - a different type of anxiety. Most two year-olds see strangers, no matter how friendly, as a threat to his safety. Your child will normally calm down as soon as the stranger goes away. However, should your child still react to strangers in this way after the age of two and a half, then it would be a good idea to consult a pediatrician.
The common fears that most children have are also a type of anxiety. For instance there is that monster under the bed, the cat next door, the toilet flushing - these fears are wide and varied and could very well have been triggered by something actually happening.
Other things that could cause your child to become anxious are everyday things such as the vacuum cleaner, the dark, or even the clowns at the circus. There is a basic fear of anything new, and what seems normal to adults can sometimes be very frightening to a two year-old. Something scary on TV could make your child anxious, or even the fact that his sister is really afraid of moths, could set him off as well.
Social anxiety is another one - this is when your child becomes intensely shy around new children or adults. Some children become very insecure in any new situation. Fortunately though, most two year-olds outgrow their shyness and timidity.