I'm really happy with this story! Spoilers for the finale, sort of.

Trust no one. Dipper accepted those words earnestly and instantly. He hadn't been the most trusting person, to begin with. Now his sister? She could make friends with a jar of dirt. But not him. He'd been burned enough by the people of Piedmont to easily believe that no one in Gravity Falls was to be believed. It took the whole summer for his mind to be changed, and a long summer it was indeed. Full of mishaps and mistrust. Dipper just had never been a trusting guy, as much as he secretly wanted to be.

In fact, Dipper was a skeptic. He found it easier to believe in gnomes and psychic mosquitos than to believe his Grunkle had merit. Several times he was near convinced that Grunkle Stan loathed him and was just using him for cheap labor. It was so easy to buy into people not liking him and doubting his own family just made him angry and sad. Why would Grunkle Stan be any different than anyone else? Why would Grunkle Stan like him any more than everyone else? Blood wasn't thicker than water. He was quick, then, to accuse Stan whenever the occasion arose, and incredibly slow to defend him. It took him a very long time to fully accept that the old man really cared about him. It took until nearly the end to fully see that, and then it was almost too late.

Bill didn't help his trust issues one bit. Once Bill was in the mix he began to doubt everything and everyone. Who else had the triangle screwed with? How many minds were altered by that three-sided freak? And then Bill took over his own body, and he began to even doubt himself. He'd already felt insecure and inadequate, now he felt grimy and used. Now he felt like his mind wasn't safe and never would be again. Now he felt like his mind wasn't even his anymore. He was terrified, feeling like he'd been backed into a corner and was waiting to be devoured. Mabel didn't understand, couldn't possibly ever understand, and it left him feeling more isolated than ever. Who could he trust, if not even himself? Who could understand?

Ford. Then there was Ford. Just as he thought his great uncle wasn't trustworthy, or even good, he found out he had two great uncles. And Ford was kind of just like him, fascinated by the paranormal and too much of a nerd for his own good. If anyone could understand him, it was Ford. But just as he couldn't trust himself with Bill in the picture, he was quick to attack and accuse Ford when he suspected Ford was Bill's puppet. When he learned that Ford once had been Bill's puppet, everything was different. He was still uneasy, but he had a partner in his uneasiness. Someone who could understand how much pain Bill could cause. He quickly clung to Ford's influence, willing to do anything for him, just because it finally felt like he had someone he could trust.

His willingness was put to the test soon enough. Ford called on him to make the hardest decision he'd ever made, and it had been so easy a decision to make in the moment.

"Be my apprentice." Absolutely! But what was he forgetting? What was he missing as he promised his life away just so he could feel safe in this scenario of mutual trust? By signing off on this adventure he was neglecting the one other person it had always been easy to trust. For a few weeks Ford had proved himself an ally and a friend, but his sister had been there with him through thick and thin for years. They'd always had each other's backs, even when they teased each other, and he couldn't just abandon that. At first, he'd thought he wanted to, but the truth was that he didn't need Ford's apprenticeship. Because with Ford, he would just keep on believing the words written in the journal. Trust no one. He would keep on being a skeptic and an outsider. But if this summer proved anything, it was that when he was with Mabel, he could be more than that. This summer he'd made friends, he'd found a family, he'd discovered that not only did people like him, he liked people.

Learning to trust people was the most important thing that had ever happened to him, and it wouldn't have happened to him if Mabel hadn't been there right next to him showing him that it was okay. Showing him that even when people did hurt and disappoint you they didn't define the whole human race. That for every person who would steal your happiness, there was someone else just around the corner looking to give it back.

Because she pushed him to trust he got back a security he'd been searching for his whole life. Perhaps what he'd craved was right there beside him the whole time, but he wasn't the first person to miss what was right in front of him, and he wouldn't be the last. If going to Oregon with his best friend was what needed to happen for him to learn how to trust again, so be it. He wouldn't trade this summer for anything in the world, even though it tested him more than anything ever had, it had made him stronger than anything ever had.

And nothing made him stronger than facing the force that had tried to take all that trust away from him. Bill had haunted his nightmares for much of the summer and would linger in his mind for years to come. Dipper had nearly lost everything alone, now, by trusting those around him and by trusting himself for the first time in a long time he was able to take on the demon. And that's when he learned to truly trust Grunkle Stan. Because he saw the side of Stan that he never had before. The side that Stan had tried to present that Dipper had stubbornly refused to see. But as he saw his great uncle sacrifice himself for his family he realized that the love had always been there. And he was appreciating it just a little too late. He didn't cry, but it was hard not to. When Stan looked at him without recognition, without the love that had been there the whole time, Dipper felt like he'd been hit by a bus. And when that recognition and love was back, Dipper was finally ready to tell Grunkle Stan that he loved and trusted him too.