Why cancer cells metastasize?

Metastasis is so complicated and well organized that there should be a good reason for it to happen. Here a hypothesis is proposed that metastasis of cancer cells is an abnormal form of migration of native stem/progenitor cells since cancer cells derive from stem/progenitor cells and may inherit stemness, including migration ability. This is an intrinsic potential and external cause mode. During metastasis, cancer cells are involved in the stem/progenitor cell recruitment to meet the need of organism for homeostasis, regeneration and repair, mediated by external signals and using inherent mechanisms but leading to catastrophic results. The "seed and soil" hypothesis can be redefined as that the "soil" is formed under certain circumstances and the "seed" is attracted to its particular "soil". Cancer cells in the microenviroment mimicking stem cell niche may have superiority in reactivity to metastatic signals. And very few of migrating cancer cells can form metastases. The conditions suitable for metastasis formation are still waiting to be revealed. The hypothesis tries to explain why cancer cells metastasize. It is hoped that the examination of this hypothesis may lead us to the real answer.