My daughter was born a social butterfly. She has made friends everywhere she goes, from a very young age. My son, however, prefers to socialise on his own terms and with familiar faces. Forcing playdates on him with children he doesn't know, isn't his idea of a fun time. He loves to play with children he knows, rather than a large group of children. He feels most comfortable when he has a close friend to play with.
I learned quickly that he needed structured play dates, while my daughter could spend an hour on the swings with a girl she only just met. I learned to plan things in advance for him, like a craft day with a friend or a trip to a trampoline park. He likes to know what to expect. He thrives on structure, while my daughter thrives on spontaneity.