Summary: When Missouri wished to enter the US as a slave state, some compromises had to be made. The balance between free and slaves states were already equal at 11 each, and the Northerners (the anti-slavery states) didn't want the balance to be thrown off, and render them inferior. In order to solve this, it was proposed the creation of a new, free state, Maine, and so on March 3, 1820, the Missouri Compromise was put into place, once more settling the balance between free and slave states in the US.
Northerner's Perspective: After hearing of a new slave state being added to the US, the northerners felt their views and say on slavery were being threatened, and honestly, they were. The north states were anti-slavery, and if Missouri was to be introduced to America as a slave state, the balance between the North and South would shift, with 12 slave states and only 11 free states.
Southerner's Perspective: The southerners knew they would have the upper hand in government if they were to add another slave state, which is why they pressed to hard to gain Missouri. The south would control the senate and be one step closer to legalizing slavery in the US.