Relatively sure it's social conditioning because pink, a derivative of red, used to be considered a "masculine" color up until and possibly including the Victorian era. Girls were typically dressed in blue because it was a soft, comforting color.

Also, as an artist, I was taught color theory in school. A basic breakdown for color use is that blue is soothing, red is angry or bold, green is growth, and yellow is agitating. I'm talking about when used in art, what those colors symbolize emotionally. And it's the same in advertising, actually, which very carefully selects colors for products that reflect those meanings.

It's interesting that we've pushed pink so heavily onto girls, and I"m curious what the impetus was. Why the colors were so drastically shifted in terms of their gender association. The truth is most little kids prefer blue unless they're told not be...because it is a soothing color.

Personally, I detested pink for a long, long, long time because I was told I must be a boy because I liked blue. I like certain pinks now that I'm older and no longer hold a childish grunge against a color.