From the Catholic Encyclopedia:
Francis's lightsomeness had its source in that entire surrender of everything present and passing, in which he had found the interior liberty of the children of God; it drew its strength from his intimate union with Jesus in the Holy Communion.
The mystery of the Holy Eucharist, being an extension of the Passion, held a preponderant place in the life of Francis, and he had nothing more at heart than all that concerned the cultus of the Blessed Sacrament.
Hence we not only hear of Francis conjuring the clergy to show befitting respect for everything connected with the Sacrifice of the Mass, but we also see him sweeping out poor churches, questing sacred vessels for them, and providing them with altar-breads made by himself.
So great, indeed, was Francis's reverence for the priesthood, because of its relation to the Adorable Sacrament, that in his humility he never dared to aspire to that dignity.