Because 520 is the opposite of 250. On that day, people who celebrate it is 250 I think.

Because lovers can't wait to show.

Because all the passion and resentment that happens in a relationship will eventually go to 0.

Maybe 520 is a number that can be divided by ten.

I think the 520 and the 521 are both okay for the young couples. They just choose which one they like bitter to celebrate!

Because 520 is earlier than 521.

Love works in a mysterious way. You never know. 

Personally I think that is because "520" appeared first compared with "521". Preconceived idea keeps a strong hold.

Because it is said that love can make people become a fool. 

I think people wants pass Valentine's Day twice.