Being from the UK where we mark who we are voting for in a private booth, on a slip with all candidates [1] fold the paper in half so no one can see and post that into a box, the Spanish system makes me uncomfortable. Let alone that people mess with the stacks of slips, hiding smaller party's slips under other piles…
[0] https://c8.alamy.com/comp/KN3EHD/a-hand-pick-a-voting-paper-... [1] https://www.unlockthelaw.co.uk/images/Ballot_Papers_General_...
I can remember as a 17 year old trying to convince my Dad to vote a certain way in a referendum which I myself was too young to vote in, and he ended up voting the way I wanted him to (although, it is entirely possible that he might have still voted that way even without my input.) I think my reasons for wanting to vote that way (despite not being allowed to) were likely better informed and better thought-out than those of most of the voters who were old enough to vote.