情不知所起一往而情深
[qíng bù zhī suŏ qĭ yī wăng ér qíng shēn]
Taken from the Ming dynasty playwright Tang Xianzu's play Peony Pavilion (Tumu ting), it means 'love, not knowing whence it came, deepens once embarked upon', symbolizing love that arises unexpectedly yet deeply impacts the heart.