Why was president tyler upset on the annexation of Texas?

Answer 1

Texas wasn't annexed, it joined the USA by treaty, which is why the state flag flies at the same height as the U.S. flag when on side-by-side flagpoles.

Tyler and everyone else was afraid that the treaty would mean a war with Mexico, which in fact DID occur within months.

Texas wasn't the only issue between the USA and Mexico, as there were problems with Southern California too, but Texas was the biggest and final straw on the camel's back.

The Mexican-American war began when Mexican artillery opened up on Fort Texas, in Brownsville, from across the Rio Grande, in Matamoros, starting a six-day siege and war with the USA.

Tyler was right.