Well, I can only relate my own experience. (we back onto a schoolyard)

My neighbors would leave their dog out from 7 a.m. until past midnight on a regular basis.
Sure the dog wouldn't bark 'constantly' all day long.
But let's say that it barks and wakes you up at 7.
Then it barks every time somebody walks past the house.
Then it barks when the kids go to school.
It barks all the way through recesses and the lunch hour. (still barking every time somebody walks past)
Then if nobody got home by evening, it'd start barking to be let in.
You'd finally get to sleep and then it'd start over again the next morning at
7 a.m.

It's only 5 and 10 minutes here and there. But try listening to that every day, all day while you're trying to write music or do something as simple as just read a book or think about something and you can see why people get so upset.

It's like mental torture and you can't do anything about it.
The police don't want to hear about it.
The animal welfare won't get involved
The local township won't get involved because there's not enough bylaw officers.
The owners won't take responsibility because 'it's impossible that my dog could have been barking all day long'

I hate to say it, but if your neighbor has been pushed to the point where he's leaving you letters, then he's just about to blow.

Seriously man.. I wouldn't laugh it off or assume that the guys just being an ass.
Personally, I only took it as far as complaining but man....I was pushed to the limits.
All I could think was, it's a good thing I'm not unstable or mentally unbalanced.
It's like, when I see headlines like, "man shot dead in front yard by unknown assailant"
or "house fire - arson suspected"
I start wondering 'hmm... wonder if the guy had a dog'