We all know it is annoying. So I searched the web. A web site said this was because our ears 'smelt' and mosquitoes were attracted to that smell. (I cannot find the web site now.) Interesting.
Perhaps it is not the smell; it could be something else. But that does not affect the hypothesis I am going to elaborate. It does not matter why mosquitoes are attracted to our ears, the important fact is that mosquitos are attracted to our ears. My hypothesis is that attracting mosquitoes to our ears is the result of human evolution, as a defense mechanism against mosquitoes.
Mosquito is most likely the number one animal that kills the most people in the world. It transmits numerous deadly diseases and unlike other deadly insects, it is everywhere. So it is apparent that as the number of mosquito-bites increases, so does the risk of getting deadly diseases. Thus, I can say those individuals who had less mosquito-bites had higher chance of survival (even if the difference were small, it could be significant when accumulated for several millions of years).
During this summer, I caught and killed a lot of mosquitoes. Often I discovered the mosquitoes because they buzzed round my ears. If they attacked my back or my leg first, I would not have discovered them and probably been bitten. In fact, those times I got bitten, I was sleeping or the mosquito bit my leg. Killing mosquitoes is relatively easy once we have found them; it is finding them that is difficult because they are silent, small, and agile.
Our ears are good for detecting things omnidirectionally, and unlike eyes, they are always 'on' even in the dark. So, as long as we are awake, if mosquitoes buzz round our ears, we have a high chance of noticing them. And if a mosquitoes is round your ear, that also means it is in the 'killing range' of your hands. So the individuals who successfully attracted mosquitoes to their ears than to their legs had higher chance of dodging the bite, and prospered. Thus, by the power of natural selection, current humans have the ability to attract mosquitoes to the ears.
Now, one might ask, "But, do our ears really intentionally attract mosquitoes? Isn't it just mosquitoes happen to be attracted to our ears?" Well, when we talk about evolution, there is no distinction whether it is purposeful or not. "Purpose" is something humans have made up. It only exists in our heads. In reality, if (for any reason) A exists and B happens if A exists (for whatever reasons), one could say A's purpose is to cause B. This is a kind of anthropomorphism to make it easy for us to understand. It would not do harm if we are not stupid. Dr Dawkins and other evolutionary scientists use this method to explain things. If you read their books, you would know what I mean.