Many of the American settlers were from Southern states and slavery was legal there. In Mexico slavery was illegal and the settlers were afraid that Mexicans wouldn't let them couldn't keep their slaves . They thought since they had purchased their land they have the rights to do what they want on their property.
The constitution in Mexico was frequently being changed. Mexico's 1824 constitution allowed Texans freedom to rule themselves. Later on they made a new constitution that gave Mexico's government more power. This new constitution made many Texans upset.
Many Americans believed that Texas should be apart of the United States. They felt as if the United States extended from Atlantic to the Pacific and in Mexican and/or Native American in between should be kicked out to make room for the "rightful" owner.