Attracted to carbon dioxide
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.
The human body odor attracts them
House flies are attracted to human body odor. This is why they sometimes leave everything and land on a human.
They are attracted to the heat of the warm body
Some flies get attracted to warm surfaces. A fly might land on a human body because it wants a warm surface to stand on.
They are attracted to strong smells such as perfume
Flies are attracted to strong smells, and the human perfume could provide enough reason for a fly to come seeking a human.
The more you sweat, the more you attract them
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 like to land on open wounds
Flies feed on dead cells. and an open wound provides a fly with plenty of supply of good nutrients.
They find food on human skin
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.
Humans are less hairy
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.
A place to poop and vomit
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?)
House flies taste with their feet
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.
Flies get attracted to negative emotions
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.