Well, this is best explained within the four noble truths:
The first sermon that the Buddha preached after his enlightenment was about the four noble truths. The first noble truth is that life is frustrating and painful. In fact, if we are honest with ourselves, there are times when it is downright miserable. Things may be fine with us, at the moment, but, if we look around, we see other people in the most appalling condition, children starving, terrorism, hatred, wars, intolerance, people being tortured and we get a sort of queasy feeling whenever we think about the world situation in even the most casual way. We, ourselves, will some day grow old, get sick and eventually die. No matter how we try to avoid it, some day we are going to die. Even though we try to avoid thinking about it, there are constant reminders that it is true.
The second noble truth is that suffering has a cause. We suffer because we are constantly struggling to survive. We are constantly trying to prove our existence. We may be extremely humble and self-deprecating, but even that is an attempt to define ourselves. We are defined by our humility. The harder we struggle to establish ourselves and our relationships, the more painful our experience becomes.
The third noble truth is that the cause of suffering can be ended. Our struggle to survive, our effort to prove ourselves and solidify our relationships is unnecessary. We, and the world, can get along quite comfortably without all our unnecessary posturing. We could just be a simple, direct and straight-forward person. We could form a simple relationship with our world, our coffee, spouse and friend. We do this by abandoning our expectations about how we think things should be.
This is the fourth noble truth: the way, or path to end the cause of suffering. The central theme of this way is meditation. Meditation, here, means the practice of mindfulness/awareness, shamata/vipashyana in Sanskrit. We practice being mindful of all the things that we use to torture ourselves with. We become mindful by abandoning our expectations about the way we think things should be and, out of our mindfulness, we begin to develop awareness about the way things really are. We begin to develop the insight that things are really quite simple, that we can handle ourselves, and our relationships, very well as soon as we stop being so manipulative and complex.