The first dog that I had as and adult, adopted me.
Betty was a doberman, and she just showed up on my front porch one day when I came home from work. I gave her some water, and some meat I had in the fridge, and she was still there in the morning. I later discovered she just loved my front porch as the best place in the world. The postman was not happy however.
After a day, I put her in my backyard, and went to the store to get dog food, and a dog bowl, and a bed. She would climb the fence, and go sit on my front porch. She was a total sweetheart and loved tennis balls. The neighborhood kids would bring over a tennis ball, and play with her in my back yard.
She was smart, and easy to train. After a short time she would see me grab the lawn mower and go toward the gate. She quickly learned to sit, and wait for me to snap my fingers. She would race to my front porch, and watch me mow, and the traffic go by. I could shut off the mower, and go dump the grass in the compost pile, come back and finish mowing, and she ran to the back yard, knowing her front porch time was over.
I never found out where she came from, she just showed up. I still miss her.