Why do cells become insulin resistant?

Cells become insulin resistant because they are trying to protect themselves from the toxic effects of high insulin. They down regulate their receptor activity and number of receptors so that they don‘t have to listen to those noxious stimuli all the time. It is like having this loud, disgusting music played and you want to turn the volume down.
You might think of insulin resistance as similar to sitting in a smelly room and pretty soon you don‘t smell it anymore because you get desensitized.