The gods, moved by Psyche's love for Cupid made her a goddess. Valentine's Day as may have started from the ancient belief that lovebirds began to mate on February 14th. February 14th was to honor Saint Valentine who was beheaded by the Roman Emperor Claudius II Gothicus for continuing to marry young couples even though Claudius forbade it. Claudius thought that married soldiers weren't as good as single soldiers. That is how St Valentine got involved.
Fertility is nature's way of making more from few and is necessary for the species.
We suspend out belief and buy into Saint Valentine's Day dedicated to lovers, sentimental greeting cards, and mushy words of never-ending adoration just because it is fun.