Generosity and happiness appear to reinforce one another in something closer to a loop than a single one-directional effect: giving tends to lift mood, and people in a better mood tend to give more readily, which means a single generous act can quietly set off a small, self-sustaining cycle rather than a one-off gesture.
The link holds up even when the amount given is small, which suggests the effect has less to do with the material value of what is given and more to do with the sense of agency and connection that giving tends to produce, regardless of the size of the gesture itself.
This is one reason generosity is sometimes recommended, gently, as a practical response to a low mood, not as a substitute for addressing more serious difficulties, but as a small, reliable lever that tends to shift how a person feels even on an otherwise ordinary, unremarkable day.