At the end of this movie, it looks as though Will is all ready to take one of the jobs that Professor Lambeau has set up for him and use his intellectual gifts to their fullest potential. But then at the last second, he leaves a note for his therapist, Sean Maguire, and takes off for California. His note to Sean says to tell Professor Lambeau: "Sorry, but I had to go see about a girl."
In doing so, he repeats the same line that Maguire used with his friends back in 1975 when he first met his wife, Nancy.
On a symbolic level, the end of this movie tells us that Will is finally willing to embrace the uncertainties of life and put his heart on the line to be with someone he loves. Throughout his life, he has pushed people away because he's been afraid that they'll hurt him emotionally if he lets them get too close. But after making a real breakthrough in therapy, he's prepared to make choices about his future that are best for him and no one else.
Finally, Will's departure shows that he's also willing to escape the little cocoon of safety he's built for himself by hanging out with his childhood friends from Southie. His pal Chuckie is certainly happy to see him go because he's tired of watching Will work construction jobs and waste his talents.
In the end, we can assume that everyone will be happy except for Professor Lambeau, who will no doubt be disappointed once again that Will has walked away from a prestigious job.