A theory (far from proved, but certainly also nothing I'd consider "disproved") is that the spike protein itself may be dangerous, in which case injecting something into yourself that causes your body to deliberately produce it may not be an unmitigated good.

I don't think we have the data either way to be sure the spike protein is completely harmless, though obviously how dangerous it is is bounded by the fact that many people experience no ill effects. (That is, it obviously can't be super-ultra-deadly, because it's not killing everyone instantly. How dangerous it could possibly be is bounded by the fact that many people have received the vaccine.)