Andrew Jackson worried that annexing (aka adding) Texas to the US would divide the nation because Texas would enter the Union as a slave state. During the early to mid 19th century, US presidents were constantly trying to balance the number of free and slave states. Their reasoning behind this is that they did not want any side (aka free states or slave states) gaining too much power in Congress.
During this time, Northern politicians were worried that having a lot of slave states in Congress would result in the slave states making slavery legal in all of the new territories gained by the US during the 19th century. The Southern politicians were worried that if free states controlled Congress, they would try to make a law/amendment to get rid of slavery.
