What makes these critters want to fly into my ears? Even when I'm wearing repellant with DEET they seem to want to go in my ears (usually when I'm putting). Also, why do mosquitos strike in places where I don't notice them till it's too late (back of knee, etc.)?

I don't have the definitive answer to your first question, but I can make some educated guesses.

First, the critters are attracted to body heat. The human head radiates a lot of heat relative to the rest of the body.

Second, the eardrum inside the ear canal is close to "core temperature," which is warmer than the outer skin of the head and ear. That's why hospitals often take your temperature using a gadget pointed into the ear. So the flies might be particularly interested in the infrared glow coming from your ear canals.

Finally, the flies might be buzzing all around your head, but you hear them more readily when they're just outside the ear canal. So there's a huge observation bias.

An alternative explanation is that the little buggers have detailed knowledge of human anatomy, physiology, and psychology, and they perversely choose to be as annoying as possible. I've often felt this is the best explanation.

skeeters look for sources of food that are easiest to get at.. The skin on the back of the knee is a tad thinner than average. It also doesn't have many nerve endings or hairs for the mosquito to tip you off that she's there.