Why Are Mosquitos More Attracted To Some People?

Scientists think it depends on the types of microbes on the skin:

The logic behind the effect is simple: Odors from human skin are essential cues that guide mosquitoes to our skin, and the microbes living on our skin play an important role in producing these odors. In fact, without skin bacteria, human sweat would be odorless to the human nose, according to the researchers, led by Niels Verhulst of Wageningen University in the Netherlands. [Bugs Love the Way You Sweat]
Verhulst and colleagues collected volatiles — the easily evaporated chemicals responsible for odor — from the left feet of 48 men. They then gave the mosquitoes a choice between each sample and a standard ammonia concentration. (The odor of ammonia is known to attract mosquitos.) They also sequenced DNA from the skin of the left foot; this gave them information on what, and how much of it, was living on the men's feet.
Of the 48 men who volunteered for the study, the researchers classifed nine as "highly attractive", while seven were considered "poorly attractive."
In addition to finding that a greater diversity of skin microbes seems to deter mosquitoes, the researchers came to associate certain types of bacteria with how delicious the mosquitoes found the person to be.
The more tasty men had microbes that were less diverse and were likely to include Leptotrichia, Delftia, Actinobacteria Gp3, and Staphylococcus microbes, the researchers found. 

Kinda gross, but interesting.  I guess that disproves my sister's assertion that she gets bit more because she's sweet (Well, that and she isn't.  Just kidding).