Zayn was viciously protective of Harry (some would say overprotective) and I think too many people in the One Direction fandom overlook that fact or downplay it because it’s an inconvenient truth to other shipper theories. He was NOT as protective of anyone else as he was of Harry. And no one else in the band was protective of Harry in the way Zayn was, especially not Louis.
See Zayn also being protective of Harry’s mom here!
Harry was also super overprotective of Zayn in a lot of tiny meticulous ways. See here:
Harry being Zayn’s personal water boy and sharing his food with only him: