While some 2 yr olds do like to interact with other kids, a lot of them are into "parallel play" - playing by themselves beside other kids who are playing by themselves. This behavior can persist in some kids for another year or two. By 4 and 5 most children enjoy playing with each other at least some of the time, although the occasional child of this age who is very shy or introverted, may still prefer solitary activities. However, if everything else seems normal, there would be no need to worry. If your child doesn't respond to being called, engages in repetitive behaviors or has a lot of anxiety, then you should ask your doctor for guidance. If he is happy but prefers his own company, don't worry about anything for now.  Sarah Chana