Joy in a dream usually reflects a lasting sense of fulfillment and connection -- lightness that shows up when something in your life is actually working, not just when you wish it were. Waking with that feeling still lingering can be worth noticing, since it often points to a source of real satisfaction you haven't fully acknowledged yet.
The aspects of the sextile or trine have the same quality: but the sextile is less forcible than the trine, either in good or evil. - William Lilly (1602.-1681.)