What causes dogs to want to bark?

Yes, sometimes they do bark at other dogs because they do want to play. But also sometimes they also my see something they are scared of. Or they are happy to see that you came home from work, or school, or were ever you may have gone. And, also, some breeds just tend to bark more. Like the Pomeranian, the jack russel tarrier, the yorkshire tarrier, and some of the bigger dogs like,like a Rottweiler, or a doberman pincher. He might just be asking for you to play with him or to eat if he is hungry. I hope this helps!!

They bark because they are 'dogs'. What makes a cow want to 'moo'? or What makes a chicken want to 'cackle'?

The sounds animals make is usually species specific.

They also have body languages. If you don't understand the body languages you are missing out on a lot.

Anger
Fear
Excitement
Somethings wrong (like I'm hunnnnngry)
Alerting you
Hope these helped :)

probably:
excited
asking for something
hungry