In Ds1 hollowing is litarly a huge thing. That you deny this is stupid.

Ds1 also had the best quest about hollowing. Siegmeyer.

And other npc hollow all the time. It's very well explained. There are a ton of theories about it. We know you can become undead by choice.

Humans have two souls. The white souls and the dark soul. The dark souls is something the gods and other creatures don't have. And if you are branded which is a ritual your humanity burns and is surrounded by fire. Hence the famous circle fire picture.

And hollowing is a result of the fire burning away your humanity. Without humanity the husk of the body reverts back to the state before the original pygmy found the dark souls. A undead.

You prevent to become hollow by willpower. Because you willpower is part of the humanity.

Crestfallen Warrior hollows due to already losing hope. And being pretty pessimistic.

Logan hollows because he achieved his goals In life. And had nothing to strive for. Which is quiete tragic.

Siegmeyer is more complex. He avoids his problems about his family and past. The adventure he seeks is escapism. And when you hell him enough he hollows because he is reminded of his past instead of the adventure.

Also ds1 is very complex and litarly gave enough lore for two additional games.