According to the constitution, the president is
required to
(1) sign or veto bills passed by congress
(2) establish income tax rates
(3) review supreme court decisions
(4) raise money for political parties

(1) sign or veto bills passed by Congress

There were numerous factors. One was for example that American settlers went westwards more and more and settled territories that were not even states but rather were Mexican and would eventually outnumber the population like what happened in Texas. Another was that there were a lot of people who supported the war with Mexico to get to the ocean. Another was possible economic power that people could achieve in a possible new state.