chant breakdown:
loser mentality - Mexico: Si se puede. Yes we can. In sports, this is just stupid. You're giving yourself underdog status, and with an insufferable and annoying team and fanbase such as Mexico you know that they think that they can beat anyone, rendering this false-humility as pointless. Losers.
winning mentality - United States: I believe that we will win. It's "we may suck, we may be good, but I think we'll win either way so fukk da haterz" - it's true humility, which rings as particularly truthful given America's perennial underdog status in soccer.
despite this, vamos mexico