I keep seeing these "+1 because I like how you ..." (freehand circle in an image, quoted a book in an arbitrary source, ...) comments, and I'm running out of flagging capabilities if this trend continues. Really, there's no need to leave a comment for each time you like an answer.
Indeed, the question shouldn't be who invented this custom, but whether it's welcome.
In my opinion this is most often done to draw attention to a particular elegant aspect of the answer. In this sense the +1 comments are helpful, especially because you don't have the time to scrutinize every answer.
Other than that I like the community building aspect of it, as Simon already pointed out in his comment. I feel this is also the reason why many people keep on coming back here. Votes are rather faceless, and knowing who voted for you can be nice.
In the case of a downvote I'd say an explanation in the comments is almost required.
I agree with you that it wouldn't be good if everyone would "+1 for..." with every vote they'd cast, but I believe that isn't currently the case and people are using it sparingly.
I find these nice, especially as someone new to SE. I find that the comments make me feel accepted by the community and that my contributions are actually being read which encourages me to make future responses.
There are some such comments that I find less useful, such as...
"@Andy Thanks for the answer!!!"
However, these don't seem to dominate the comment landscape and are generally from first-time posters who are trying to be polite.
For some common ground on the issue, we could propose some etiquette for removing such comments after a certain amount of time or after the intended recipient has had a chance to see it? I don't really know how that would or could be managed.
The FAQ is clear about this: compliments are not recommended, and "+1, great answer!" is given as a specific example of undesired behaviour.
While I also think David should have talked about this in the chat, as he is not asking a question but complaining, I don't understand why he gets downvotes. The FAQ has a section concerning comments, and as we're in beta, it should be discussed.
The OP should consider editing the question to allow for more constructive debate.