Although you might wanna be careful who you kiss, because they can transmit a number of diseases, kissing a healthy partner can be good for your teeth. There are a few reasons for this:
1. Everyone has bacteria in their saliva; about 80% is common to everyone, and 20% unique to each individual. When you exchange another healthy person's saliva, you are exchanging the 20% unique bacteria that they have in their mouth. This means that by exchanging saliva, you are being exposed, and become resistant to that 20% of unique bacteria!
2. Passionate kissing stimulates the flow of saliva. This neutralizes acids, and flushes away food particles, preventing tooth decay. More saliva also means that your mouth will be less dry, and dry mouths are more prone to infections.
3. Saliva helps rebuild tooth enamel, which further prevents tooth decay.