They fell in love with each other at the same time, but neither of them forgot that they already had a lover. If their lover could resist the temptations of this world and stick by their side like them, they would definitely have always been ordinary friends, or they would have become passers-by in each other's lives.
Many fates in this world begin with liking and end with staying together. If they didn't persist so much, how could they have such undying lovers like each other?