Why do flies sit on humans?

Some flies are attracted to the carbon dioxide, which humans breath out. As a result, they might circle the head then decide later to land on the human.

House flies are attracted to human body odor. This is why they sometimes leave everything and land on a human.

Some flies get attracted to warm surfaces. A fly might land on a human body because it wants a warm surface to stand on.

Flies are attracted to strong smells, and the human perfume could provide enough reason for a fly to come seeking a human.

Flies are attracted to sweat, and so the more a person sweats, the more likely that a fly will land on him. (See What Attracts Mosquitoes to Humans?)

Flies feed on dead cells. and an open wound provides a fly with plenty of supply of good nutrients. 

Salt, sweat, proteins, dead skin and oil are among the foods flies consume. Flies find plenty of that food on the human skin, and that's why they like to land on humans.

Flies can find it harder to extract food from hairy animals. Because humans are less hairy, a fly can easily be able to extract the food it's looking for.

Flies usually look for places where they can poop and vomit. A fly vomits on solid food to liquefy it and make it more soluble, and because the human body has lots of food for the fly, it might seem like a good choice. (See Why flies are so annoying?)

Because house flies taste with their feet, they have to stand on whatever they plan to eat first to find if it's delicious. A fly can stand on a person to taste the excess nutrients on their skin.

According to one theory, flies can get attracted to negative emotions because they result in more sweating. A person who feels anxious or afraid might sweat more and so attract more flies to him.