So, on my third playthrough, I was tired of doing Concord, and didn't really want Power Armor. So I skipped it. I skipped the Red Rocket station too. And Sanctuary. I just headed South and started exploring.
I go through Boston, but skip Diamond City. The first real settlement I entered was Goodneighbor. Which leads me to the Memory Den .... you can USE the Memory Den if you haven't started the main quest. Color me shocked. So after a rather horrifying trip down memory lane in a lounger, Irma apologizes, and in sympathy, tells me about a friend of hers that could help me track down some answers. A private investigator in Diamond City. Nick Valentine.
So I travel to Diamond City, now with an actual defined goal, and go through the quest of saving Nick. Once back at his office, we do the interview, and head to Kellog's house. Now, remember, I'd never met Dogmeat yet. There is a full scene and extra dialogue from Nick to introduce him, as a talented dog that hangs around and occasionally helps Nick out with stuff. Dogmeat really doesn't have an owner, but he and Nick have an understanding, and Dogmeat will respond and come to Nick after the synth puts out a special high frequency "dog whistle" call. (This also explains why Dogmeat just shows up out of the blue if you had already met him but didn't bring him with you to Kellog's house.)
Can I just say that this make SO much more sense than randomly finding a super-trained dog in the Wasteland, or following the psychic drug visions of an old woman?
It REALLY makes Concord and Red Rocket look obviously tacked on - just to start the game with an exciting fire fight in Power Armor, and give you an "ahh, look at the puppy" companion immediately. The story events for the whole first act work MUCH better with more logic and flow without them, but it looks like Bethesda was terrified to just let the player wander with no direction at first.