Interestingly enough, Lego has become my son's favorite toy as well. I had no influence over this development, other than purchasing him a small Star Wars Lego set when he turned four. Since then, he has become an avid Lego builder, and even suffers withdrawal symptoms if he does play with his Lego sets for a day or more. It's fascinating to watch him sit, sometimes for hours on end, clacking the blocks together as he manifests the visions in his imagination. When I see him playing Lego, I am also reminded of other benefits playing with Lego imparts upon children - patience, independence, problem-solving, and the list goes on.
Even though stepping on a Lego block in the middle of the night remains one of the most painful things a human being can experience on this planet, it's a small price to pay in exchange for this jouet extraordinaire.