Ordinarily speaking, we say that we are happy when we get the thing we are longing for. At the time when we desire something, our mind is in a state of unrest; but when we get that thing, the state of unrest is replaced by a sense of peace. Similarly, we say we are happy when we get rid of a thing which we loathed. At the time when we loathe something, our mind is restless; but when we get rid of that thing, our mind becomes calm. Therefore, happiness follows when a preceding state of unrest is ended. Happiness is identical with the feeling of rest or peace. Only he who knows mental peace experiences true happiness. And to still the mind is the aim of all spiritual practices.
To paraphrase: true happiness is peace of mind.