It all started the day she saved him from that incident in the river. He remembered that day with such vibrant clarity, both due to the nature of his near death experience and prospect a new-found friendship that had emerged out of that faithful day. For you see, the Lorax's plan to safely sail the Once-ler out of the valley, had well... gone kind of awry. Instead of using his bed to sail him down the river and out of the valley, the Once-ler found himself bobbing amongst the white waters of the river. Struggling to keep his head above the water the Once-ler saw a shadowy enter the water and make its way towards him, as the darkness engulfed his senses. Suddenly, he was pulled away from a blinding light and from out of the darkness his eyes opened to revealed to her face. That was all it took. From that day the two developed a mutual friendship, that has blossomed into something more.
(Planning to make this into a series of 3 drabble oneshots featuring young-Once-ler, greedy-Once-ler, and finally old-Once-ler. Be on the lookout for more.)