This quote tells us the importance of empathy, of finding the good in people, and of showing mercy to one another. In the first line, the narrator is seeking to feel for himself the sorrow that others feel.  When we are able to feel what others feel, that is called empathy, and this is an important way of connecting with one's fellow man.  How can we ever get along with others if we cannot understand what they feel? This is one of the most important aspects of being human, how we grow, really, as people. The second line is about how we regard other people. The narrator is saying he wants to be able to overlook the faults and flaws in others, that he wants to focus on what is good about them.  This is an important quality, too, since no one is perfect, and we would have few if any relationships if all we were looking for were faults in others.  We can learn to appreciate the goodness in anyone, learn to overlook the flaws.  These qualities are important foundations for the next two lines, about mercy, since if we have empathy and we overlook faults, we are less likely to judge people, which allows us to have mercy for them.  In this sense, I would say that the quality of mercy is a combination of kindness and forgiveness.  It is based upon an understanding that no one is without fault, that there is some goodness in everyone, and that we should all afford one another this kindness.  As you can see in the last line, the narrator hopes that he will be afforded mercy, too.