A valentine is a person that you want to spend every day with, especially valentines day. If you dont spend valentines day with your valentine (for example, if it was your sister's birthday and you were forced to go see her and not the girl that you really wanted to see) you would be a wreck trying to find ways to apologise to that "special someone" and trying to think of exactly what would be the right thing to say. Then you might end up doing something strange like posting something on a website at 4 in the morning to try to show that person you care. Then after reading what you put on the website, that person may think you're completely insane and never talk to you again. Or they could make your day. We'll see what happens.
So, here it goes. Megan, will you be my valentine?