For the ordinary trust relationship, such as in an iterated prisoner's dilemma or exchange or in a trust game interaction over a relatively narrowly defined range of issues, you and I might share limited interests and might not genuinely care much about each other's general welfare, but we can still trust each other because we need each other's contributions to some common effort or production. In this case our interests are weakly shared through the causal effects of our efforts on our joint good. For the lover and the parent, the interests of the other are much more strongly shared: they are directly included in the parent's or lover's own interests to some substantial extent because the lover and the parent care directly about the general welfare of the other. Therefore, the other has strong reason for trusting. The range of matters of concern to the lover or parent is typically very broad; for the ordinary trust relation, it is typically narrowly defined.

Russell Hardin, Trust and Trustworthiness, loc. 661