What's the cause of my 5-month-old kitten developing an obsession with suckling me, anywhere on my body?

Your kitty was taken away from its mother (or lost its mother) at a very young age and she has not outgrown suckling. Even after spaying, there is a good chance she will still want to do the same thing. She will probably outgrow it, but no guarantees. You can either enjoy it, or gently put your hand around her snout and move it away, then give her a light tap on the nose with your finger -- this is what mama does when she wants the kits to move away from the nipple and to wean them.