I rarely cry for my own sake. What I do cry over is the suffering of others. I can stoically bare my own grief with almost no problems whatsoever, but as soon as I see a mother crushed under the weight of her grief for her son that passed away, I can bare it no longer. This is wholly due to my capacity to empathize, the ability to understand and share feelings with another. Empathy works in a very interesting way, actually causing us to feel the pain and sorrow of others we come into contact with. When we see someone in pain, the same parts of our brain that are associated with actually feeling that pain become active.¹
This is almost certainly an adaptive trait, forged over millions of years of evolution in order to foster social interactions between individuals that benefit our "selfish" genes. But, even though the "motives" behind empathy are selfish, we can in fact hijack this useful capacity and use it as a tool for our own purposes, namely moral reasoning.
Empathy is not the foundation of moral reasoning. The foundation must be reason itself, in my opinion, and not a subjective emotional response to stimuli. But, interestingly enough, I think that empathy, even though it is purely affective, actually can be an extremely useful tool for objective moral reasoning. Empathy informs us that another's misery and happiness is as valuable as our own in a very tangible manner. It is a window that allows us to look outside our scope of self-interest and peer, however imperfectly, into a truly impartial world where misery is miserable and happiness is joyous, no matter who experiences it. This is something that could only be done in the abstract without the capacity to empathize, which explains why most sociopaths don't see things this way. They lack this extremely valuable tool, and are thus forced to live inside their scope of self-interest, with not a single window of which to look out.
I, for one, am glad I can look out the window, even though it makes me cry sometimes.