My son does the same thing sometimes. He's three now. He used to be afraid to make the first move and started out very shy. Now he sometimes is quiet just to be frustrating but we (and his preschool teachers) make him talk before we give him what we know he is looking for. He's gotten much better and now approaches adults and other children with a bit more confidence. Overall he's had positive interactions so I think that's helped build some confidence. He also sometimes just likes to watch other people and not get involved so maybe that has something to do with it. Anyway, hope that helps a bit.