Probably for the same reason you do -- to get information about things they don't understand. Some toddlers also love the adult attention they get during conversations and continue asking why as a way to keep you engaged. As irritating as this can be, keep your cool and answer as many questions as you can. Use some of your adult conversation skills to steer the discussion away from a Q & A (try asking him what he thinks the reason for something is) and into other areas your child knows more about. Or you can simply turn the tables and ask him a few "why" questions you know he can answer. Of course if you don't know the answer to one of his inquiries, tell him the truth, but explain that you can find out together. Have your child sit with you at the computer while you search for the answer online, or plan a trip to the library.