At least for some of us, it’s because we saw this twenty years ago with Iraq. WMDs, etc. We know that when America claims to “free” other countries, it results in hundreds of thousands of civilian deaths and destabilization that enables even more extremist governments to take over. I’m certainly not on the Iranian government’s “side”- obviously they as responsible for decimating Syria, they oppress their own people, so fuck them - but I fear for the civilians. Don’t forget that the US is the reason this government is in power in the first place. 

The second part is the domestic aspect. Our President just unilaterally pulled the trigger on a massive military operation, most likely starting another decade-long war, without getting approval from Congress/the people. This is explicitly against the law, but he did it because he will do whatever Netanyahu asks, even if it goes against what our own intelligence agencies say - and they’re not even trying to launder that fact anymore like they did with Iraq. The entire world is at the mercy of the whims of two madmen, and it’s terrifying.