Giving someone a gift is fun. Giving someone a gift that is a total surprise is really fun. Giving someone a gift that is a total surprise and then watching their expression change to "What in the hell?" is not fun.
Note to self: Your brother does not play in the dirt, therefore buying him a matching Tonka Toy Backhoe to go with the Tonka Toy Crane you bought your nephew is probably not a good idea. Actually, it's a rather bad idea.