North Elbonia attacks South Elbonia. Why should the US use US money and US supplies and US lives to support either non-US side?
Because failure to support South Elbonia against North Elbonian aggression will lead to a more dangerous and less free world in which the US will be both poorer and at greater risk.
You see a mugger attack someone weaker than themselves. You are stronger than either party. Why
wouldn't you defend the victim against the attacker?
Any party that initiates violence against another party is a risk to the entire community, and as a member of that community you have many good reasons to intervene, not least your own selfish desire to live in a world where you are less likely to be attacked. This is true of the community of nations just as it is of your own local community of individuals.
I'm astonished that anyone could reach adulthood without understanding this.