The Slavery Issue, Chaos in Mexico, and Manifest Destiny lead to the war that gave Texas it's independence.
The Slavery Issue
No esclavos (No Slaves)
I bought this land! I should be able to do whatever I want with it, including having slaves.
Chaos in Mexico City
That government idea will never work! There are flaws!
Why have their been so many Americans passing by?
Settlers coming in from America had mostly been white male plantation owners. This means they owned slaves but they weren't allowed to have slaves since slavery had been outlawed in Mexico. Some Americans ignored this.
In Mexico there were many debates on how the government should be formed, it was always changing so it made it difficult to settle problems without a proper central government.
Americas believed that they had the divine right to move west. They wanted their boarders to be the pacific ocean and the Atlantic ocean so they wouldn't allow anyone to stand in the way.