Charlie notes from the very beginning of this story that he wants to be smart. Although he lacks the intelligence of even an average man in society, he understands this deficit and wants to have an opportunity to become more intelligent. Charlie is driven by an internal motivation to excel, and from the start he wants the doctors to choose him for their experiment. In their tests, the doctors realize that Charlie has a good "motor-vation," as he calls it, and they tell him that this isn't a common trait for people with an IQ of 68. Charlie doesn't even know what an IQ is.
Charlie believes that being smart is somehow linked to an ability to have friends. There is a theme of loneliness that is woven into the plot, and when he lacks intelligence early in the story, he doesn't even understand that sometimes people use him as a joke; this is particularly evident in his comments regarding Joe Carp and Frank Reilly. Before the surgery, Charlie comments that people who are smart never get lonely and have lots of friends whom they can talk to. This demonstrates his sense of isolation; people don't reach out to engage with Charlie because of his low intelligence.
Charlie's desperation to find acceptance and friendship propels his desire to become "smart" because he believes that this will solve all his problems. Unfortunately, he finds that humans are more complicated than this, and becoming intelligent doesn't erase his feelings of loneliness.