I'm interested in hearing others' answers to this one. My personal take on it is a firm 'no, it's not an obligation', but it's been a while since I actually thought about the issue, and I'm not sure how much of my reaction is reflexive defensiveness. (I know that I work better when I don't feel obligated-by-society, but that's not much in the way of evidence: My reaction to feeling manipulated or coerced far outweighs my reaction to feeling obligated.)