A glazed donut is a pastry with a shiny coating of sugar covering it. The basic idea is that if you put someone’s body-part in your mouth, for the purposes of this discussion, let’s call it a “long finger”, when it leaves your mouth, the wetness gives it a shiny look like it was glazed.

So to say you are glazing someone would mean you are such a fan of that person, you would be willing to make some parts of them really, really shiny with your mouth saliva.