I know you're quoting Aristotle or some other famous Greek writer, but I only partly agree.
None of us knows what life will bring us; none of us is able to predict how our lives will end, although we all know it will end.
Since that's true, I think the Buddhists & the old Greek and Roman Stoics and the author of the Book of Ecclesiastes have the right take on happiness.
Since we know we have to die, and we don't know what good and bad life will bring us before then, it's essential to be "happy" or at least "joyful" NOW, at this moment.
The Buddhists & the Stoics mostly believe that to achieve joy in the moment, despite the inevitability of death and pain, etc., it's necessary to cleanse the soul/mind of desire, or at least inappropriate desire.
Only when we can transcend our insatiable desire for pleasure rather than pain, etc., can we achieve the detachment needed to open our souls to lasting joy, they suggest.
Many westerners have a hard time accepting this notion. But I think we need to move in that direction if we are to achieve happiness that endures. We need to accept the impermanence of the self, the prospect of our own inevitable mortality.
To the extent we can, we need to open ourselves to the beauty and/or love and/or good of each moment, so that we don't forget to harvest our happiness when it is abundant -- even if the only happiness we can grasp is from a beautiful sunset, or a quiet and peaceful moment in bed, or a good deed performed successfully, or a nice time with a friend or lover or relative.
We also need to remember that death is "always sitting our our left shoulder," as anthropologist Carlos Casteneda once put it in one of his "Don Juan" books.
Because death is always sitting on our shoulder, and because it could snuff out our existence tomorrow, every instant we are alive becomes more important & a bigger occasion for happiness.
"Little birdy, little birdy, why do you sing your song?
It's a short time I've got to be here,
and a long time to be gone."