It was an accident, or so he said.

John had been messing around with something and asked you to bring him a small blade, and in the process, he'd nudged you, causing you to accidentally slice your finger and a few drops of your blood to fall into the jar he was mixing up.

There'd been a puff of red smoke and a somewhat stunned, but barely disguised joyful expression appeared on John's face.

"Oh, love..." He says slowly, glancing up at you, "That was... Ah, you have to promise not to freak out." He voice was deceptively soft, even as he stood up, immediately going to his shelf of books and flicking through one.

"I'm afraid we've been magically tied. Think of it like a... A witches marriage of sorts! ... And uh... That wasn't meant for you." John's lying through his teeth, but you don't know that. "And... I can't reverse it. But, it's okay, it's okay! The only thing that's really changed is that we share magic now! Isn't that nice?"