Gratitude is built into the calendar here so it cannot be forgotten in the good years. A man sows, waits through a season he does not control, and reaps. At the turning of the year he stops and hands back the first and best of what came up. He offers the firstfruits before he has proof the rest will hold. That gesture concedes what pride keeps trying to deny, that the harvest was never entirely his doing.
The field returns grain because someone worked it and because reality met the work halfway. To mark the ingathering with a feast is to refuse the lie of the self-made man without denying the labor. You worked, and you were given to. Both are true, and the celebration says so.