I think you misunderstand. It's NOT the custom for a Pope to resign willingly. Those that have left office and have not died in it (a VERY small list), were kicked out due to a political coup.
Except for Celestine V, who was a very pious man who had been the reforming abbot of an order of hermits before he was elected Pope, in hopes that he would do something to reform the Papacy and the Church (which was, as usual, seriously out of order). It was too much for him and he resigned. He was later canonized for his piety and the work he did for his order.
However, Dante placed him in Hell for having shirked his responsibilities when he was called to assume this burden. The "Great Renunciation," Dante called it, IIRC.