“Friendship may, and often does, grow into love, but love never subsides into friendship.”
George Gordon Byron (1788–1824) English poet and a leading figure in the Romantic movement
Quoted by Marguerite Gardiner, Countess of Blessington in Conversations of Lord Byron with the Countess of Blessington http://books.google.com/books?id=w648AAAAYAAJ&q="Friendship+may+and+often+does+grow+into+love+but+love+never+subsides+into+friendship"&pg=PA179#v=onepage (1834).