Why do moles cause cancer?

They can change! Melanoma is a skin cancer that involves the cells which produce pigment. Moles on our skin also contain the same cells so when they change and appear worrisome (the abcde's) we worry about those cells transforming into melanoma. So moles don't cause cancer but can transform into cancer given the right conditions (sun exposure, family/genetic history. Etc..). Visit www.Skincancer.Org for more info.
They dont. Moles do not cause cancer. A skin cancer can develop within a mole or present as an abnormal looking mole.