Arthur Schopenhauer said this quote

Why, then, does the man in love hang with complete abandon on the eyes of his chosen one, and is ready to make every sacrifice for her? Because it is his immortal part that longs for her; it is always the mortal part alone that longs for everything else. That eager and even ardent longing, directed to a particular woman, is therefore an immediate pledge of the indestructibility of the kernel of our true nature…”―from_The World as Will and Representation_. Translated from the German by E. F. J. Payne. In Two Volumes, Volume II, p. 559