I find it interesting and pleasing that "Happiness" is capitalized in this quote. To me that signifies that it is a Spirit-given, inherent, built-in truth of our being, a quality we can access like peace, joy, and love. It's something we are born with.
As we know, we can get off center on all those inherent qualities, far from "steady and stable." The key suggested in the quote is being content with what we have, rather than being in a constant state of wanting. Wanting takes us into the future, and the unsatisfying state of "I'll be happy when I get ________." There is nothing wrong with desires--we can't live a human life without them--but if having everything we want is a requirement for our Happiness, we are bound to suffer disappointment. Having everything we want is a constantly moving target.
I have good health, food, shelter, love, the opportunity to love, and satisfying, useful work. Hmmm...I guess that's why I am usually in a state of steady and stable Happiness.
Content with life,