An overemphasis on rights leads to a disregard for duties. Duties set a limit on rights because they point to the anthropological and ethical framework of which rights are a part, in this way ensuring that they do not become license. Duties thereby reinforce rights and call for their defense and promotion as a task to be undertaken in the service of the common good. (CV §43)
Jesus commands us to love our neighbors. It is therefore our duty to do so, and our neighbors consequently have the right to expect that love from us. This way of looking at things is reinforced throughout the Old Testament’s repeated declarations about the duty of the rich to have mercy on the poor.
In Caritatis in Veritate Benedict clarifies the relationship in social justice between the duties we owe our neighbors and their rights. If we talk about rights to the exclusion of duties, we will surely find things imbalanced: lots of talk about what people need and have a right to receive, and a deficit of the goods necessary to meet those needs. This will and does happen because when we fail to talk about duties to the poor, we readily get the perverse idea in our heads that we personally don’t have any duties to the poor. By the same token the failure to fulfill one’s duty will often lead to others’ (namely and usually the State’s) usurping that duty. Resentment often follows: how dare they take away our hard-earned money only to give it to a bunch of lazy so-called poor people?!? This is one likely practical fruit of fulfilling our duties to the poor: the government will stop trying to do the job. That would be an enormous good, given the fact that they do the job so badly.
Really, though, the best reason to fulfill that duty is because we love our neighbors.