This is one of the hardest questions when it comes to a personal theology, that is, a theology that incorporates and nurtures the idea that God cares about each person. There are many theories. One has already been suggested, that the evil in the world exists as a balance to keep good and bad in perspective. That is to say that a rose only smells good because we know how bad sewage smells. Well, the problem here is that this goes against it's own principle of God caring about everyone. What about those who are the subject of the evil acts, does God care about them too? How can he, when he allows such terrible things to occur to those people?
Another theory is that God will give everyone their "just reward" once this life is over. I am not sold on the idea of an eternal afterlife so this doesn't really work for me either. I have a tough time with this because it places less emphasis on this life and makes it less meaningful. There are many others but these are the two that I can think of off the top of my head.
But if you were to pull me aside and say "Timeless, with all the is wrong in the world, how can we know that God cares about us personally?" I would say that I do not know. I am not using this as a cop-out or as an excuse not to look into the topic. Quite the contrary, by stating that I do not know it opens my mind to any and all possibilities. In the end, no one really "knows". Those who claim to know base their ideas on personal experience and ancient texts. I am not sure I am ready to put stock into such a shaky source. Personally, I try to keep an open mind and continue to move forward in my understanding the best that I can.