After sharing some of my headcanons on the GF Amino, I got the motivation to write a full story based on one of those loose ideas. Here, my friends, is how I imagine Dipper got his nickname.
"Isn't it beautiful?"
Mr. Pines looked from the sky to the awestruck faces of his twins, getting his answer. He may have been a computer guy, but something about the stars always fascinated him. He looked back up. You just didn't get these views in town; you had to drive out of the city. So he'd taken the four-year-olds out of Piedmont to show them his favorite part of a clear summer's night. Now, here the three of them were, nestled on a blanket in the grass, Mabel on one side and Mason on the other.
Mr. Pines pointed east. "See that one? That's Hercules. He's a big, strong warrior, and he's gonna go over there and fight that one," he pointed the other direction. "That's Scorpius, and they don't get along too well."
"It's so pretty!" Mabel cooed.
Mr. Pines chuckled. "I know, right? Oh, and Mason, you'll like this one." He pointed again. "That's Draco. He's a dragon, like the one in your favorite story."
"Cool!"
"Hey, look at that!" Mabel pointed straight above the trio. "I saw that one before!"
Mr. Pines followed her finger and saw... Ursa Major.
"Oh that? That's Ursa Major. He's a big, wise bear. You'll always see him; just look for his tail! It makes another constellation, called the Big Dipper."
"Dipper," Mabel repeated. "Like Mason?"
"What?" Mr. Pines looked at her.
"Dipper." She nodded to her brother, who was still studying the sky.
Mr. Pines looked from his daughter to son. "Yeah... Dipper..." he said, noting the odd birthmark he barely noticed anymore. He smiled and hugged both twins closer.
"Daddy?" Mason asked.
"Yeah?"
"Can we come back soon? To look at more stars with you?"
"Of course."
"Good." Mason smiled and gazed thoughtfully at the sky, snuggling closer to his sister. "Because I love it."
Mabel looked back at the Ursa, then her brother. She would remember that one, not for the reason her dad said, but because it reminded her of her best friend.
"Me too, Dipper."
Fun fact: every constellation mentioned in here is actually visible in summer. I did my hokework ;)