Some of these were because it was an ill-treated dog.

Most of the time, however, it is just circumstance. Last weekend, I was jogging on a forest path, rounded a corner and suprised a dog whose owners probably considered it a well-behaved pet-- and the owners looked liked 'responsible members of society' (i.e. not trying to prove they were tough).

The dog was off-leash.

Because both I and the dog were caught off-guard, this immediately turned into an aggressive situation. The dog was threatening to attack me, and the owners had a very hard time getting it to back down-- it wasn't responding to their voice commands, it kept barking and lunging at me.

long story short, from my perspective, any domestic animal weighing above 4kg should always be on a leash when out of the house, as a matter of public safety.