This was the first time I've ever given up my seat for anyone on the subway (although I did once on the bus) and not because I don't want to. I have a couple of problems with doing it. For one, when I'm riding the subway, it's often early in the morning and I'm barely conscious, much less looking around the train to see who I'm riding with. And two, I struggle with being afraid of offending people who are borderline old enough to need a seat. I sometimes see people in their late fifties or sixties who I think should be sitting down, but whom I think are still active enough to potentially be offended by the implication that they're too "elderly" to stand for a few minutes.
I've seen many a seat given up for someone else, be it an older person, a pregnant lady, someone with a cane, or even sometimes just because a man wants to be chivalrous towards a woman.
I love seeing it because it's a quick reminder of how good people in this city can be.