Source identified1
“When one is in love one begins by deceiving oneself. And one ends by deceiving others.”
The line turns romance into a two-stage comedy of self-deception and social deception, exaggerating cynicism to expose how desire distorts judgment.
Source note: A Woman of No Importance, Act III; Lord Illingworth answers Gerald’s question about love with the romance/self-deception epigram. The exact catalog wording is supported by this play, not by The Picture of Dorian Gray.