The Beatles sang "I get by with a little help from my friends."
Although this (somehow) isn't one of the many songs Charlie mentions in The Perks of Being a Wallflower, it sure does apply to his life. In fact, Charlie finds himself battling some pretty dark moments of depression, and he might not have found his way out without his friends. On the other hand, you could argue that Charlie wouldn't have found himself there in the first place without that same lovely help from his friends. If he remained isolated and alone, he wouldn't have had all those fights, break-ups, and other depression-inducing incidents.
That's the double-edged sword of friendship: can't live with it, can't live without it. And you know what? Charlie's friends might think the same about him. He's not always the most supportive companion, but he does what he can, and some intangible something keeps them all together.