Answer :
Mexico wouldn't negotiate.
Manifest Destiny.
Pacific Port.
There is many more but these are the ones I remember
[1] US annexation of Texas (December 1845), which had been part of Mexico prior to becoming independent. This outraged Mexico.
[2] US openly declared ambition to acquire California and New Mexico (then part of Mexico). The US offered to buy these territories from Mexico, and to pay compensation to US citizens with unsettled claims against Mexico for property damages.