Probably you are a vampire ! Mygoshhh ;)
Maybe you're iron deficient?
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This makes you a bored teenager who needs a hobby.
Or Blood fetish.
I was going to say something flippant, but having read this comment I'll refrain. If you actually do have a blood fetish, you should seek out information on how to indulge safely, as well as read up on the psychology of such fixations. I know from personal experience that there are groups and publications devoted to this that aren't run by psychos. Seek them out. (For the record it's not my personal kink but I've attended a seminar run by an MD who was into it).