I once stood up for an old guy in an extremely crowded train and he looked at me like I was a freak (the good kind), actually he damn near got a tear in his eye.
I guess it's because it's more of a dog-eat-dog kind of world nowadays. That and most of the time you don't think about giving up your chair, not because you don't want to or something, but if just never crosses your worried mind.
I've never stood up for a girl though... even if I DID like her, standing up for her isn't goanna magically win her over for me.
Maybe if I asked if she wants to sit on my lap?