"Love?" the Doctor sniffed, nodding at the paper heart in the window. "A chemical reaction. Nothing more. What makes one particular individual 'love' another and vice versa? Hormones and personal standards - conscious and subconscious - based on gender and sexuality issues, and the relative mental and physical attractiveness of the other person."
Peri rolled her eyes.
"However," the Doctor continued, "in the spirit of the season..." He waggled his fingers, and plucked a beautiful red rose from thin air. "Happy Valentine's Day," he said, offering the rose to Peri with a smile.