and, Why do We Celebrate Valentine's Day?

Good question.

The holiday was established by Pope Gelasius in 496 AD and named after Christian marty Valentine. Thus, St. Valentine's Day.

If we were to get into the details, then it should be St. Valentines' Day (note the change in punctuation?) because there was more than one Christian martyr named Valentine.

Valentine lore has it that the first Valentine's Day Card was sent in 1415 from France's Duke of Orleans to his wife when he was a prisoner in London Tower.

One more fun fact: about 8 billion candy hearts with fun lovey-dovey sayings were made in 2009. Yum!