It turns out the little buggers are adept at sensing our body chemistry and decide who smells best.
According to an article on webmd.com,
Genetics account for about 85% of our susceptibility to mosquito bites. Certain elements of our body chemistry, when found in excess on the skin's surface, make mosquitoes swarm closer. People with high concentrations of steroids or cholesterol on their skin surface attract mosquitoes. Mosquitoes also target people who produce excess amounts of certain acids, such as uric acid, according to entomologist John Edman, spokesman for the Entomological Society of America. These substances can trigger the mosquitoes' olfactory sensations, or sense of smell, causing them to launch their "landing" onto unsuspecting victims.
But the process of attraction begins long before the landing. Mosquitoes can smell their dinner from an impressive distance of up to 50 meters. This doesn't bode well for people who emit large quantities of carbon dioxide.Who are these people? Look around. If you see someone scratching bites, he's probably a candidate.