Acceptance of circumstances is one of the key Warrior virtues. The Wise Sage understands the difference between a circumstance and the true self. He does not judge events—no “good” or “bad”—but rather accepts all situations out of his control and allows himself to feel appropriately in reaction. No matter how he feels or what life brings him, he stays true to his self awareness—the knowledge that his true self does not change, that he is whole and pure at his core. And so, in non-resistance to life’s circumstances, he also accepts himself.
In this life, it is not possible to ALWAYS be happy. Circumstances will arise that will make you sad, angry, etc. And that is okay. The point is, that it is okay to feel any emotion, and non-resistance to—acceptance of—happiness or sadness, as well as the situations which arise to incite such states, ultimately brings what IS possible to achieve: A lasting state of peace.
The Buddha instructs us to stray from judgments, such as “happy” or “sad,” and rather focus on finding a non-attached state of inner harmony, whereby one allows oneself to feel anything he or she feels, and be okay with it, without judging. Hence, don't project life’s goal to be a constant state of happiness, because that is an impossible task and a set up for disappointment. Instead, focus on allowing all feelings to come through, and be totally at peace with who you are and what you're feeling, no matter what chaos the outer world seems to bring. That way, you'll be able to learn from experiences and emotions, rather than judging your feelings and reacting without thought. Let peace and understanding be the goal...as opposed to the idea of "happiness."