We love Lee, but it's okay to admit that he's a little too focussed on his friendship with Elle. It's a major point of contention between him and Rachel in the second film, and his utter devotion to Elle is the main reason the best friends bump heads in the first movie. Courtney told POPSUGAR that he wants Lee to grow as a person in the next instalment, saying "I'd love to see Lee just come into his own and be completely confident. I want to see him land in himself and just be able to hold his ground in life."
Depending on how things play out in the third film, Lee could be going off to college alone. Rachel never reveals where she's going off to, but let's assume she won't be attending Berkeley. Without his best friend, his brother, or his girlfriend, Lee would finally be able to be his own person — a fresh start. He would learn to stand on his own without the security of Elle right by his side, which would be a true moment of growth for him. It would also help him learn how to detach and not devote so much of his energy to just one relationship. He could learn to juggle close friends and romantic partners; which he needs help in!
And if Elle or Rachel do decide to attend Berkeley with Lee, the chaos of college is the perfect remedy for codependency. Do you think you have time to be all up in each other's business with a full course load and manageing a social life? Barely.