When I let my dog out into the garden, he barks at me for a while. I usually shut the door and he runs off to do his business, without looking back at me or barking at me again. I'm not sure why, is he just excited? He does want to be let out. Maybe he's paranoid and wants me to come out too? He's been on the alert since he saw a cat in the garden. He's a cavapoochon.
It's most probably a vocalization of his excitement.
Barking is not always something negative or aggressive. Compare it to a human screaming: it can be someone screaming in rage or frustration, or it could be a child screaming in joyful play.
It could also be his way of broadcasting his presence to any potential intruder or his dog-friends in the neighborhood. His way of saying "Here I am."
When I let my dog out into the garden, he barks at me for a while. I usually shut the door and he runs off to do his business, without looking back at me or barking at me again.
He's inviting you to come out and play. When you shut the door, you have declined the invitation so he stops asking - a very polite dog!
This is common between dogs as well. When a lively dog approaches another in the park, does a play bow, and gets no response, the next level is to bark. At this point the owners usually intervene but with an inexperienced or over-confident dog it can escalate into nipping. At that point the other dog might retaliate. If the second dog is dominant, the barking dog will retreat. If they are equals, a brief snarling match (or even fight) may ensue. If the second dog is old or weak, the barking/nipping dog may start to nip more and harass the second dog.
Your dog is asking but not insisting. Incidentally does he do or sketch a play-bow when he barks? (front part of body repeatedly lowers and bounces up again while barking) or does he stand stock still and just look at you while barking?
If he bounces, that is a request to play. If he just stands then it's a request simply to come outside and keep him company.