While commuting on a train, I would and do happily surrender my seat to someone who I think needs it more than I do, such as old people and pregnant women, there are cases in which I would hesitate. Two cases where the chivalrous act is likely to cause embarrassment to or even offence of the receiver are: (1) when the 'old' people, particularly women, do not think they are old enough to be pitied, and (2) when the women are not pregnant but just a bit thick around the waistline.
On the BBC website today is an interesting article about this painful dilemma. An interesting figure quoted by the article is that 84% of the pregnant women regularly have to stand, one of the reason being that "seated commuters don't want to offend the non-pregnant".
So you see, it's not necessarily that the seated people are selfish.
To give or not to give, that is the question.http://news.bbc.co.uk/2/hi/uk_news/magazine/8730106.stm