it's impossible to tell who is normal in these stories.
for example, the guy can start smoking inside, and it will always smell like smoke.
or, the guy can get his own buzzer.
i've had oversensitive neighbors sit there and bang on pots all night because fireworks or construction noise.
like another comment mentioned, apartments are just built really badly. you can hear anything. which leads to friends of mine complaining about stompers, but to me, they're clearly not stomping. they're just tip-toeing around as quietly as possible. when you get people actually harassing you daily, then you figure out the difference.
if you have to set up a machine or device, then you might be the bad neighbor. this is especially true if they don't set up a machine back at you. that means they're just taking in your harassment and not escalating.
people aren't total idiots. they figure someone is messing with them for their level 15 volume and keep it lower. For anyone with a TV, try putting your volume to 15. Is is unreasonable volume? It's very hard to say who is the psychotic neighbor. I've been on both sides: neighbors that were loud but normal (maybe they had bad hearing, or worked at night), and neighbors that were oversensitive, who were petty and bought machines and devices and sprays to really hurt all of the people who lived around them.