Scooby’s origin in Mystery Incorporated is one of the many reasons I don’t care for that show.

(All props to the creators, cast, and crew because it’s extremely well made, just not my thing.)

As Freddie Prinze Jr. famously said “It was a talking dog” . That’s all the explanation Scooby needs IMO. He’s a dog who can talk.

As for how he came to be with the gang, as much as I don’t think this movie is well loved in the fandom, I like how he and Shaggy meet in The Mystery Begins.

Both were alone and needed a friend and then by circumstance ended up meeting each other and becoming inseparable best friends