Why did Shaggy pick Scooby over Velma?

Because a dog is a man's best friend.

A classic case of "bros before hoes." He made the honorable choice. He stuck by his buddies side and didn't let anything get between them.

He wasn't good at romanic relationships like Fred and Daphne. He loves Scooby and food too much. Most of all, He's a man's best friend.

Because a dog is a man's best friend.

Shaggy and Scooby are kindred spirits.  

Cuz Shaggy is a dumbass.  He and Scooby could have been tag teaming Velma all these years.

Maybe he likes Daphne? Or maybe he's an off screen stoner who thinks Scooby can talk.

In my opinion, Shaggy made the right choice. He picked Scooby because Scooby is a certified homie to Shaggy. He has been with him through thick and thin, and there is nobody else like him.

Your best friend for years who you feel totally comfortable with vs a girl who constantly puts you in uncomfortable situations and makes demands that you just don't want to do? Shaggy made the right choice man. 

he's a furry