What were some of the many warning signs that Andy was extremely depressed?

Andy clearly exhibited symptoms of major depression in the novel. 
For example, Andy was constantly plagued by feelings of guilt and hopelessness. He told his coach after a winning game that he should have been the one who died. Later, he admitted to feeling guilty for taking over Robbie's position on the team.
He told his psychologist that he would never have won the position of center had Robbie been alive. Andy thought that Robbie was the best center Hazelwood ever had, and he thought that he could never match his friend's prowess on the court.
Andy was also easily irritated and exhibited extreme mood swings. For example, he could be cheerful one moment and belligerently angry the next. During an interaction with Keisha, Andy complimented Keisha on her looks, which prompted Keisha to comment on his apparently good mood. Yet, a few minutes later, Andy became extremely angry when Keisha mentioned that she had to study for her chemistry test and could not go to the movies with him. 
Andy also experienced frequent bouts of sadness, which left him feeling deflated and apathetic. For example, Andy became melancholy after seeing a Santa Claus display. He told Keisha that Santa Claus reminded him of Robbie. Keisha admitted that she did not see the connection, as Robbie was African American and six-foot-five when he was alive.
However, Robbie immediately shut down, refused to talk, and ignored her. Keisha related that she had to call her mother to take them both home, especially since Andy no longer drove. Later, Andy told Keisha that he never turned in one of his school assignments. He admitted that he did not care if his grades suffered.
There were many warning signs that Andy was extremely depressed. He exhibited apathy, experienced frequent bouts of sadness, struggled with feelings of guilt and hopelessness, and displayed extreme mood swings.