Why did Texas declare independence from Mexico in 1836, and why were many Americans reluctant to accept it as a new state in the Union?

At the time, new settlers were being encourage by Mexico in a bid develop their economy and also increase their demography. Although slavery was illegal in Mexico, American settles entered Mexico with their slaves. Till this time, a large fraction of Texas population came about from the Union.

Americans were reluctant to accept Texas as a new state because they were afraid that their slaves would be freed by the then President of Mexico. Sectionalism was a huge and crucial factor of American politics as that the time. In this light, free states possessed a higher number within both arms of the government. Thus associating Texas to the Union could possibly mean that there may be reversal of power.