To be happy, we have to be willing to love: to know, recognize and be fully drawn in by the good.
Then our desire prompts us to act in order to possess the good, and we rise to the challenge with the adrenalin kicks of hope and boldness.
And when we possess the good we seek, we come to a joyful rest.
Not only can we enjoy the pleasure of the presence of what we love, we can reflect on ourselves being joyful, bringing us even greater joy, because as Aquinas says, the "thing is perceived all the more perfectly the more abstract and non-physical the mode of perception" (1a2ae. 35. 2-4). This self-awareness of the presence of the good is a clue as to why beatitudo ends up being the ultimate happiness.
passion and evil
Passion first and foremost drives us to seek the good, but it also rejects and repels what we perceive as threatening or harmful. Just as we naturally love things good for us and ours, we naturally hate things that are evil (e.g., bad luck, negligence, flaws, distortions of form, evil perpetrated by others), especially when it is threatening or harmful. Obviously, if we hate something, we are repelled by it and feel despair if it is overwhelming. We are also aroused by fear or anger to avoid it, and if the evil befalls us, we experience sadness proportional to the harm done. But both sadness, when expressed, and anger, when it combines with the desire and hope of righting the wrong, can act to move us out of the avoidance of evil and back into the pursuit of good, which is right where we belong.
passion and will
According to Aquinas, it is better to seek the good with all we have, including passion, as long as it is in sync with our will actually seeking the good (since obviously not everything we desire is truly a good). And if we do act well, we should enjoy it: "Good actions can't be perfectly good unless completed by pleasure in the good done." (1a2ae. 34. 4)
The question then becomes, if we want to attain the pleasures of acting well, how can we dispose ourselves to make the right choices?
"We get more pleasure from union with the spiritual objects
we understand than from the bodily objects we sense: the objects
themselves are to be prized more, the ability to understand is
a nobler ability, and the union achieved is more intimate, more
complete, and more lasting."