Most of us only put in as much effort as a situation requires from us. If we can 'get away' with being less considerate or less reciprocal, and various other forms of 'getting without giving,' many of us will, not because we're evil, but simply because we can. If people demanded or expected more of us we would do more, but when they don't, we don't make the effort. This dynamic is true in practically every relationship we have. When our self-esteem is low and we expect very little of others, we are likely to get very little from them as well.