Here's why:

* Uncontrolled Cell Growth: Cancer cells lose the normal controls that regulate cell division. They divide uncontrollably, forming a mass of cells called a tumor.

* Mutations: Mutations in genes that control cell growth and division can lead to cancer. These mutations can be inherited or caused by environmental factors like smoking, radiation, or certain viruses.

* Tumor Formation: As cancer cells continue to divide, they form a tumor. Some tumors are benign (non-cancerous) and do not spread, while others are malignant (cancerous) and can invade surrounding tissues and spread to other parts of the body.

It's important to note that while cancer is characterized by rapid cell growth and division, not all rapid cell growth is cancerous. For example, normal cell growth and division is accelerated during wound healing or pregnancy.