During the Andrew Jackson administration a rising number of Americans migrated to Texas and formed settlements their. It was for this reason that Jackson was in favor of the annexation of Texas. This position of Jackson's would later cause trouble for the Martin Van Buren Administration.
Jackson was a Democratic-Republican. Despite changing the structure of the party and changing its name to the Democratic Party, Van Buren essentially supported the political decisions of his predecessor. He supported his economic decisions, his decision to place Federal funds into private banks, and his belief that Texas should be annexed.
Americans who lived in Texas were excited about the idea. However, those who lived in free states weren't so thrilled, as Texas would have been a slave state. So, more so to stave off a split within his own democratic party, Van Buren announced in 1837 that he did not support the annexation of Texas.
It should be known here that he did not make this decision because he opposed slavery: he made it merely for political purposes.