Missouri wishes to enter the USA as a slave state.
Missouri as a slave state will throw off balance!
Missouri and Maine are now US states.
Missouri wanted to join the union as a slave state, and received support from preexisting slave states, since the addition of Missouri would cause them to hold the majority.
But a few northern congressmen did not wish for Missouri to join that way, because with slave states holding the majority, there wouldn't be negotiation and communication going on within congress.
So then Henry Clay wrote up the Missouri Compromise, which convinced enough people to grant Missouri admission to the USA as a slave state, by also admitting Maine as a free state, and prohibiting slavery to the north of Missouri's southern border.