Well, I disagree. They have no reason to puff up unless they feel stressed. It is a defensive thing, but we can't always figure out quite what they think is a threat. So I would never recommend just going and chasing the puffer around until it puffs up, which people often want to do. In itself, the puffing is not a stress, it is the stress that must be inflicted before it feels threatened enough to take this evasive action.
I don't think they just do it because they like it! We just don't think like fish, or may have missed the nip from another fish, etc...