(But I'll give it a go regardless.)
It's because, regardless of intent, war is hell, and eventually everyone will know someone who has had an arm blown off as collateral damage, or they'll read about the fifth wedding ceremony this year to have been accidentally vaporized by a cluster bomb. Everyone will either have been directly affected by, or at least know someone directly affected by, the ongoing civil war instigated and perpetuated by a foreign power. I mean, can you imagine if the Chinese tried to do nation building the US? No matter how bad it were to get in the States, people would resent that and they would be right to.
And it doesn't help that the US has a well-deserved reputation, in the Middle East and elsewhere, of propping up failed states run by ruthless, brutal dictators. You have no moral high ground to claim. None.
But honestly, this is just a basic, common sense thing. Your military should engage the armed forces of other countries when those armed forces pose some existential threat to you. In no other case is it appropriate. If you don't already know this it's because you are willfully ignorant to it. So, please just fuck off.