No matter how clean you think your mouth is, our saliva can carry up to 1000 different types of bacteria. Human saliva has gram-negative bacteria, which birds do not naturally have. A bird's immune system can not defend against gram-negative bacteria, this is were kissing your bird could become dangerous.
If a bird was to ingest even the smallest amount of our saliva it could make them very sick and it has caused deaths in birds.
Another way you could make your bird sick from saliva is from lipstick or gloss. Although safe for human kisses it's definitely not worth the risk your bird ingesting lipstick or saliva.
Sharing your cereal with your bird is also a common way birds can get sick from ingesting your saliva. Especially from spoons or other utensils, you have put in your mouth.
It's not only birds that can get sick from kisses! Birds can also carry a disease called Psittacosis. Which is a fever-like cold which has even caused illness and death in humans.
A great way to reward your birds for great behaviour or just to let them know you love them is a wholesale diet and lots of enrichment. A few treats on hand helps as well.