I'm American, but i have a serious preference for self-checkout because I can avoid tje annoyances of regular check out. Waiting on the person who forgot their wallet. That person who wants to argue about one dollar on a one hundred dollar purchase. The person who realized they bought to much and now needs to figure what they want to put back. And more.

Cashiers are nice, I guess. But stores are not a social call for me. I want to buy my stuff and not deal with ridiculous people in front of me.

Also self-checkouts just move faster. This probably wouldn't be true if stores had the same number of cashiers they used to, but it really reminds me of mu youth where you could be in and out of a store without a significant wait to check out.

I lived in France for a bit and the stores were completely different. Speedy and plentiful lines, so that's probably the difference between Spain and the UK