apart from all the reasons you people gave (which may be all true in some way imo^^), I remember him talking about it on an episode of domoto kyodai...if I remember correctly, he compared it to the typical veils which always hang infront of ramen-shops and such places, he said something along that if he takes his glasses of, it's like welcoming friends^^
I could search for the clip but I'm not sure if it's still on youtube because I think they deleted alomst every domoto kyodai-episode..