Gnostics were a strange bunch. Their oddity is intimated in the wild divergences of their practices. For some, the valuation of spirit over matter, immateriality over corporeality, signaled a license to baccenalian festivity. A careless abandon into the worthless flux and fancy of material's happenstance delights.
Others, however, saw the dualism as indicating a caution. The purity of spirit required that matter's pollution be avoided by all rigor and force. Fasting and abstention from all physicality was the mechanism of purification.
The Christian tradition, along with Aristotle, has long recognized these two as the two basic movements of the sinful soul. License and legalism name these two worlds. Nothing new here.
The phenomena that requires explanation is, however, why does man mostly bend toward feast and not fast?
The explanation is that God is Father, Son, and Spirit. God is a God full of life, a community of festal love. He is light and in him there is no darkness at all.
As images of God, even in our sin we copy him.