The enigma surrounding joy is this: that while most desirable, joy is not to be sought for its own sake. Ironically, while most desirable, it it is not what is most desirable. Joy, you see, is the first fruit of love, a warm effusion of the fire of love embracing the beloved. We might call joy a 'light shadow', the bright aurora or halo of love which always accompanies true love but which cannot exist without love. Accordingly, it makes more sense to search for the enduring tree than the passing fruit, it makes more sense to search for the fire than its surrounding glow, it makes more sense to search for the Beloved than for the 'light shadow' of his joy to fall across our heart. If we seek the cause, the effect shall surely be ours, but he who seeks the effect alone seeks in vain. Love is the cause of joy, but love itself has a 'cause', or perhaps we would do better to say that love has a 'because', that is, a motive, which is the very goodness of the thing or person loved.
The lover perceives the goodness of the beloved and wants to be one with him. Hence, Pascal's observation: "The heart has its reasons which the reason knows not." This is quite distinct from the deceptive, though subjectively accurate saying: "Beauty (goodness) is in the eyes of the beholder." This may seem 'true', but objectively it is as far from the truth as fool's gold is from real gold, as far as bad wine is from great vintage wine. The desire is in the heart, but the good and the true belong to the order of reality. This is discovered by those, who were initially inebriated with a false illusion. Upon waking back into reality, they realize that they have been drinking the bad wine first and last. In this desire for a false good there is certainly no lasting joy.
The angels are connoisseurs of the wedding feast of God, and it is they who can school our spiritual palate to correctly discern and properly seek the goodness of Him Who alone is good. And then alone shall we know true joy that cannot be taken away.
Issuing from and following upon love, joy is the first passion of the soul and the first affection of the will. Joy is nothing less than the satisfaction or delight of the will together with the esteem with which we regard and savor the good of our desires (cf. St. John of the Cross. Ascent III.17). "Taste and see how good the Lord is!" (Ps 34,8)
(The essay contains much more on the topic of joy)