Why does my cat lick my lips every time he purrs?

Should I make him stop?

While I don't think it's imperative to make him stop, it would probably be better if you did.

1. The most concerning reason is such a behavior is unsanitary.

There are diseases like cat scratch fever that you can catch from a cat's saliva having contact with your mucus membranes, such as those in your mouth. Experts advise you to wash your hands after just petting a cat, so you can imagine getting licked on the lips is much worse. But on the other hand, realistically I think most cat owners don't go that far, and it's possible even with perfect compliance to still catch an illness from your cat, so it's up to you to assess the risk level you're comfortable with.

2. Many cats also bite when they groom as well, so I would also be concerned about possibly getting bitten on the face. Though to be fair, if he hasn't done that so far, I think it's less likely your cat will suddenly start doing that.

3. I would also expect getting licked on the lips quite regularly to not be something anyone would be too thrilled about.

Why exactly is he doing this?

It really is an unusual behavior, so I cannot think of a particular reason why your cat has become so fixated except that every cat has its own quirks and habits it picks up.

But there might be a discernable reason if you wear makeup or lip balm on your lips. Sometimes cats are inexplicably drawn to hygiene or beauty products, and will try to lick or eat them. If such is the case, he might stop if you simply stop wearing that, or at least take it off when you're at home and sitting around.

How do I make him stop?

1. Redirect him if possible.

I think redirection would be the most ideal solution if you can figure out what might work. Unfortunately, the behavior is so odd, I'm not sure what could possibly serve as an appropriate substitute. But if you discover such an item, keep it on hand while you sit, and use it to physically block him from going for your face, and hopefully he will go for that instead.

2. Gentle negative reinforcement.

If you can't figure out a way to redirect him, then this is one of the few cases where I expect negative reinforcement will be effective because he can't sneak off to do the behavior while you aren't looking. But the negative reinforcement can be very gentle, to be as kind as possible to the cat. For instance, I think gently restraining him when he comes towards your face will be enough. Most cats hate being restrained, and so this is enough to be a negative reinforcement. And when you feel he's no longer straining to come towards your face, immediately let go. If you're very consistent, hopefully he'll get the message that coming for your face just isn't going to work.