Because summer doesn't last forever.
Dipper was more upset then he expected to be leaving Gravity falls, true, they would be coming back next year, but that's a whole year, dipper would be 13 by then, and he wanted to see all the mysteries that gravity falls had to offer.
Unfortunately, summer had a nasty way of ending too quickly, just when he was finding the real mysteries, just when he was actually becoming friends with Pacifica, everything was going well, until it was time to go home.
So here he was, standing at the side of his great uncle's car, having just said goodbye to Wendy, and soos, Dipper had to practically drag Mabel to the car, she was upset to find out that her parents would not want a pig as a pet, she did not want to say goodbye to waddles "GRUNKLE STAN YOU BETTER NOT EAT HIM!" Mabel shouted to her great uncle, "but I got the clean pan, for the bacon!" he replied.
"Don't worry Mabel, he won't eat waddle's, he knows how special that pig is to you" dipper said to try and calm down his sister. "He better not!" she replied. "Right kids, you all buckled up, ha-ha, buckled up, ha, that's funny" he said, laughing at his own "joke"
"Since when was safety a joke?" dipper said. Dipper was about to get in the car when a voice stopped him in his tracks. "Don't I get to say goodbye?" that was the voice of none other than Pacifica northwest. "Didn't think you'd want to" Dipper replied. "Why wouldn't I want to say goodbye to the mystery boy?" Dipper blushed, mystery boy was kind of a nickname she had given him, and he didn't need to tell her that dipper was already his nickname.
"Well then, so long" he said. "So long? Really, worst goodbye ever, honestly, do you even know how to say goodbye to someone that you won't see for, what?. A year?" Pacifica said, shaking her head. "Yeah, a year, until next summer" he replied, he thought for a moment, then he took off his hat, and gave it to Pacifica. "Think you can watch it for me until next year?," the hat could be something he only wore to gravity falls, to put on a thinking cap, so to speak.
"Sure, I mean, yes, yes I can" Pacifica said. "Well, goodbye" Dipper said, now he thought about it, he wasn't very good at goodbyes. However Pacifica was determined to outdo him at goodbye, so, with that, she kissed him on the cheek.
"Bye" she said. "By...bye" Dipper said, blushing, he got into the car put on his seatbelt, and the car's engine started. He waved goodbye to gravity falls, and a slightly blushing Pacifica, one thing was for sure, Dipper couldn't wait till next year.