I would really like to answer Neiro here. That song bring tears to my eyes. Like seriously. Those first 7 notes, turns the ON button in my eyes and I'll be a weird person here, getting all teary. But yeah, its not XD
So the song would be Special Happiness~ ♥ It's nostalgic because, when I became a KAT-TUN fan, one of the albums I downloaded first is Best of KT and the song grew on me. I even burn it in a cd so that I can listen to it when I drive. Now, every time I listen to that song, it just made me remember the first year I became a KT fan. It has a smooth and calm melody. I always try to sing along with it. lmao