佛說盂蘭盆經疏 1.6

1.6

初、酬宿因者，悉達太子不紹王位，捨親去國，本為修行得道報父母恩。然菩薩用心不務專己，故開盂蘭法會，以福自他二親。此經所興，本意如此。

First, as for repaying past causes: Prince Siddhārtha did not succeed to the royal throne, but renounced his family and departed his country; fundamentally it was in order to practice the path and attain enlightenment so as to repay the grace of his parents. However, a bodhisattva in setting his mind does not strive exclusively for himself; therefore he inaugurated the Ullambana Dharma assembly in order to bring blessings to his own and others' two parents. That this sūtra arose, its original intent was like this.