The mind is connected with moon, and full moon is a symbol of completion, of celebration, and of a pinnacle. So Guru Poornima is the day when the disciple wakes up in his fullness, and in that wakefulness, he can’t but be grateful. So he is full of gratitude.
This gratitude of not the gratitude of you and me, or of dvaita (duality), but it is the gratitude of advaita (non-duality). It is not a river moving from one point to another point, but it is an ocean moving within itself. It is that gratitude.
See, a river moves, but it moves from somewhere to somewhere. And an ocean also moves, but it moves within itself. So gratefulness of Guru Poornima symbolizes that fullness of the student, or of the disciple. And the disciple, on this day, celebrates in gratitude.
~ Sri Sri Ravi Shankar