Partly because I was "only happy when it rains" way before Garbage thought it was cool enough to write a song about it. ;)
Seriously, I've always preferred rainy days - I could just stay in, draw and listen to music w/o my dad yelling at me that I was turning green and needed to go outside in the sun. And that remained.
But mainly, I guess, as a teenager, for a while, I was obsessed w/ the Beatles song of the same name, I'd make drawings w/ that song title all over them, and it became sort of a signature. I don't remember exactly how, but it stayed.