People always expected something in return, the thing that changes is that you no longer get it. In a small community, you're likely to meet the person you helped again, and thus leaving them stranded will definitely bite you. Not so in the current huge cities.
You say 'if I helped someone the only thing I'd expect in return would be that if the person I helped is ever in a situation where they can provide help to someone else, they'll remember the time they were helped by a stranger and take action', but is this sufficient motivation for you to actually go out and help people?
