In a friendship, there needs to be understanding, honesty, and acceptance in the relationship. Even early on in their friendship, Patrick and Sam were very accepting and didn't exclude Charlie. For example, "Patrick and Sam didn't just throw around inside jokes and make me struggle to keep up. Not at all. They asked me questions." Patrick and Sam made Charlie feel welcome to be their friend. They accepted him as he was and were genuinely interested to get to know him. Being accepted makes everyone feel good about themselves. It boosted Charlie's self confidence and allowed him to become more comfortable and social. This is something that definitely helped Charlie to adjust and fit in to his high school.
Besides being accepted into Patrick and Sam's friendship, Charlie needed to actually feel like he belonged. But before them, Charlie had never felt that before. "I just remember walking between them and feeling for the first time that I belonged somewhere." For a long time, Charlie had never felt that he belonged anywhere until he met Patrick and Sam. There's nothing like knowing where you belong, especially to a person like Charlie. It shows how close the three of them were and how happy Charlie was that he had found friends like this. Without these friends, he would have felt out of place.
The guidance of Charlie's loved ones was crucial for his growth as an adolescent in high school and for his dealing with a lack of belonging. What Patrick and Sam gave Charlie was something that nobody else could give him: acceptance. From welcoming him in with no hesitation, they allowed Charlie to feel sure and confident in himself. Patrick and Sam gave Charlie friends to lean on and have fun with, but they also gave him a sense of belonging which helped him through the hard years of high school and adolescence.