An excuse to buy her a present.
Something that she might bitch at you for forgetting.
A good day to pig out on cake and/or make out.
If you're in high school, a motive to skip homework and go play lazer tag.
Another walk of life celebrated by a useless holiday.
A contest to see how many months more you can stand one another than another couple.
Just another way to express undying love, or some similar feeling of mindless infatuation.