It’s a pretty complex subject, not gonna lie, but in my opinion, the movement of people is about as close to natural human behaviour as you can get.
Mass migrations have happened throughout history and have (arguably of course) made societies better for it.
The problem is the argument always ends up boiling to such tropes as ‘we’re full, they’re coming to claim benefits, they don’t integrate (except when they try to integrate with our women), they don’t contribute, they want to take us over,’ etc etc.
We have ended up dehumanising people searching for a better life. When this starts happening, we can enter very dangerous waters