Well, to start with, I would ask them what they are honestly trying to accomplish by telling me "Life isn't fair"...

Do they want me to give up? Do they want me to join them in their own misery so they don't feel so alone? Are they just bitter and spiteful about the notion of seeing other people enjoy happiness that they were unable to attain for themselves?

I would say to them, "You're right, this element of life *isn't* fair. So why should we take no action in response to the unfairness? How does it benefit the world to just pout and stew in a gloomy self-fulfilling prophecy?"