Why would European countries extradite american criminals to the US? Because we established a trust in each other and want to keep it that way for both sides benefits.
this is a poor comparison. Extradition treaties exist and contain specific legal obligations. It is not based on trust and there are several pairs of countries that do not have specific extradition treaties.
How are these treaties enforced? All international treaties are ultimately based on trust. There is no higher authority, only elective councils of and voluntary commitment to procedures (a.k.a. promises) by sovereign states.
Specifically not even these formal promises have been given by e.g. the United States of America which to this day has signed but never ratified either the VCLT[1] or the VCLTIO[2], so is figuratively giving a lukewarm "let's see about the convenience of that when it comes up".
