If you’ve got the same setting on for your Mac, it’ll do this as well if you’re not around any network you’ve previously joined:
So to stop this and get rid of those annoying pop-ups, you’ll visit Settings > Wi-Fi on your iPhone or iPad and then toggle off “Ask to Join Networks.”
On your Mac, that option is available under System Preferences > Network > Wi-Fi.
Turn those off, and you’ll no longer be prompted to join any networks automatically! Which is especially awesome if you’re like me and unwanted dialog boxes make you irrationally angry. Yes, yes, I know that’s not healthy.