Life is funny. The biggest joke in life is that, after all that has been done in the pursuit of happiness, it turns out that sustainable happiness is achievable simply by bringing attention to one's breath.
If you practice bringing a firm and gentle attention to your breath, after a while you may find yourself in a state where you are alert and relaxed at the same time. If you practice often enough, you may even become able to bring your mind to that state on demand.
Imagine you have a pot of water full of sediments, and imagine that pot is constantly shaken and agitated. The water appears cloudy. Imagine that you stop agitating the pot and just let it rest on the floor. The water will become calm and, after a while, all the sediments will settle and the water appears clear. This is the classical analogy of that mind in the alert and relaxed state. When we bring the mind to that state, we temporarily stop agitating the mind the same way we stop agitating the pot. Eventually, the mind becomes calm and clear, the same way the water appears calm and clear.
There is an extremely important quality of mind in that state that is not captured by this analogy. That quality is happiness. When the mind is calm and clear at the same time, happiness spontaneously arises. This mind becomes spontaneously and naturally joyful!
But why? Even after I found myself able to access that mind on demand, it didn't make a lot of sense to me. Why should a calm and clear mind automatically be happy? I put that question to my friend, Alan Wallace, one of the Western world's top experts in the practice of relaxed concentration (a practice known as Shamatha).
Alan said the reason is very simple: Happiness is the default state of mind. So when our mind becomes calm and clear, it returns to its default, and that default is happiness. That is it. There is no magic, you are simply returning the mind to its natural state.
Alan, in his deep wisdom, said that to me in his usual calm, joyful and understated manner. But to me, that statement represents a profound life-changing insight. It implies that happiness is not something that you pursue, it is something you allow. Happiness is just being. That insight changed my life.
To paraphrase a famous Zen saying, you are already happy, you just have not yet realized it. Such a funny insight.