If I'm on a train I'm in my own little world generally, usually with my head in a book. Tap me on the shoulder and I'll jump up straight away to give someone my seat, but I don't analyse every passenger stepping on the train to determine their need for a seat being greater than mine. It's not entitled to want to sit down if you are struggling, but it is entitled to expect people to be paying attention to your every move (or bump) without bothering to ask.