Because love spells are wishing for something that is against someone else's free will.
How would you like it if someone made YOU love HIM? What if it was the ugliest, fattest, guy in your class, the one with acne all over his face, the guy that smells like sour sweat no matter how often he showers?
I'm against love spells because you don't really know what you're asking for. You're asking for someone to like you no matter what else they may feel. You're asking for someone to give up their right to choose you. You're asking for a handsome prince to fall madly in love with you because the fairy godmother said so. THAT JUST DOESN'T HAPPEN THAT WAY!
More than likely, he'd find out you cast a spell, and the spell would break then and there. Because of the sevenfold return, he'd hate you. He'd tell everyone what you did, and THEY would hate you. You'd probably go years without having a boyfriend because of it.
Spells are not some light little "Wish upon a star" sort of thing! They're to be taken seriously, and you have to take responsibility for what you cast.
Please... figure out exactly what you are asking, because you're not asking if these spells work. I really think you want to know how to make a guy like you---or you're looking for someone to give you permission to cast a spell.