Dr. Edmund Bruce-Gardner

The knee is a pretty badly-designed joint. Evolution doesn't move towards perfection, but function. 
As bipedal (two-legged) creatures who evolved from quadrupedal (four-legged) ones, quite a few compromises and fudges have been made. 

[The shoulder is another good example. it is a heavily-modified hip (i.e. ball-and-socket joint), but one where the bony shape and supporting ligaments have to allow for such a large range of motion that it relies on the surrounding muscles for nearly all of its support and integrity, as well as on the function of all the other joints surrounding it.  You can read more about the weird and wonderful world of the shoulder complex here.]
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But anyway, the knee...

It is a hinge joint, the largest joint in the body, sandwiched in between the two longest bones in the human body, the femur (thigh bone) and tibia (shin bone). 
This is a fairly bad idea to start with, because long levers generate a lot of force. 
Worse, it's not a very simple hinge joint. 
If it were, and could only bend forwards and backwards, and we wouldn't be able to walk on broken or uneven ground. 
So we have the longest bones (/levers), generating a huge amount of force, going into the largest joint, which is also extremely complex.

Back when we put all of our weight through four ‘knees’ instead of two, this was less of an issue. 
​We also tend to weight-bear with our knees more or less unbent. This provides a lot more opportunity for trauma. Contrast this with other mammals such cats, dogs, horses, chipmunks, etc. etc. 

​​The bottom of the thigh bone is a bit like the cartoon version of a bone, with two knobbly bits (called condyles) at the bottom.  These then (theoretically) meet, or articulate, with the relatively flat top of the shin bone.

This works about as well as you would imagine. 

Evolution’s workaround here was to put these two sort of cups, called menisci, on the top of the flat bit of the shin bone (rather poetically called the tibial plateau).  These allow the knobbly condyles to sit a bit more firmly on the top of the tibia.  

Now, of course, we have another structure that takes a lot of force, and can get injured. 

Most sports fans (not to mention players) will have heard of a torn meniscus.  I tore mine when I was about fourteen, and still remember it as one of the most exquisitely painful experiences of my life. As you can see in the diagram above, when the menisci (the 'C-shaped' things around the outside) are in place they cover most of the top of the tibial plateau.

​But wait! There's more!  It's not all about the menisci.  In a flash of evolutionary genius, the other main way that the knee is stabilised is by four... ...elastic bands,
Well, not literally, but they might as well be. 
The rubber bands (known as ligaments by the medical types) are actually just thickenings of the capsule that surrounds the joint. 
Again, sports buffs will probably recognise the terms medial and lateral collateral ligaments.  These refer to the ligaments on the inside, and outside, of the knee, respectively. 
The anterior and posterior cruciate (or 'cross-like') ligaments go between the femur and tibia, and can be seen (along with the menisci) sandwiched in there on the left.
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​So we have this large, unstable joint subject to huge forces, that we hold in biomechanically-compromised positions.