Because it takes the full weight of your body and any extra force when you run or jump.
Let's also assume that your knees only hurt after you run, and not for too long after, a few hours perhaps. I assume these because this will mean that you are not arthritic, nor that you have sustained some sort of sports injury requiring extensive rest or even treatment. It is the hardest and therefore worst surface for your knees we have. If you can find track, do your intervals there. If not, use asphalt. If you can, find a grassy surface and run on that once or twice a week. It takes more effort to run on such a yielding surface, but it's much nicer on your feet and knees. Knee pain, especially during running, is usually due to a dysfunction in the hip from above or the ankle/foot from below the knee. Think of the knee as a junction between 2 different stilts. If one does not work right it will put undue stress to the knee.  As the name suggests, runner's knee is a common ailment among runners. But it can also strike any athlete who does activities that require a lot of knee bending -- like walking, biking, and jumping. It usually causes aching pain around the kneecap. 1 Repeated bending or high stress exercises such as lunges and plyometrics can irritate the kneecap joint (patellofemoral joint). 2  Overstretched tendons (tendons are the tissues that connect muscles to bones) may also cause the pain of runner's knee. 3  Direct trauma to the knee, like a fall or blow. 1 This can cause pain and injury to the joints. 2  Sometimes, the kneecap itself is slightly out of position. 3  Problems with the feet. 4  Runner's knee can result from hypermobile feet (a condition in which the joints associated with the feet can be move more than what's normal), fallen arches, or overpronation (flat feet). 1 Repeated bending or high stress exercises such as lunges and plyometrics can irritate the kneecap joint (patellofemoral joint). 2  Overstretched tendons (tendons are the tissues that connect muscles to bones) may also cause the pain of runner's knee. 3  Direct trauma to the knee, like a fall or blow. Introduction. Sudden pain in one of the knees is usually the result of overusing the knee or injuring it. In many cases, you don't need to see your GP. The knee joint is particularly vulnerable to damage and pain because it takes the full weight of your body and any extra force when you run or jump.  Symptoms of runner's knee are: 1  Pain behind or around the kneecap, especially where the thighbone and the kneecap meet. 2  Pain when you bend the knee -- when walking, squatting, kneeling, running, or even rising from a chair. 3  Pain that's worse when walking downstairs or downhill. 4  Swelling. 1 Repeated bending or high stress exercises such as lunges and plyometrics can irritate the kneecap joint (patellofemoral joint). 2  Overstretched tendons (tendons are the tissues that connect muscles to bones) may also cause the pain of runner's knee. 3  Direct trauma to the knee, like a fall or blow. Knee pain from running can be located in front, on the outside or back of the knee. The knee is anatomically a complicated joint, so pain affecting it can be because of a number of reasons. These range from overuse and actual injuries, which can be treated, to benign growths, which must be removed.  My Knees Really Hurt From Running. A woman is holding on to her sore knees. Photo Credit lzf/iStock/Getty Images. When your knees really hurt from running, your body is trying to warn you that something is wrong. Don’t ignore it; if you keep running, you’ll risk causing further damage. Go to a doctor for a diagnosis and treatment so you can get back to the sport you love.  Even after taking a few days off, the knee pain from running seems to come right back, sometimes even intensifying, after the first few miles of the next run. The knee pain often feels worst when running downhill or walking down stairs, and the knee is often stiff and sore after sitting down for long periods. 
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why do my joints hurt when i run