No one in the play is very psychologically oriented or astute, and that includes the town doctor, Doc Gibbs. That's as it should be, considering when the play takes place. No one knew much about the various forms and sources of depression, or about bi-polar disorder, or the myriad mental problems that we have so many names for (and pills for) these days. Doc Gibbs says that Simon has seen "a peck of troubles," and he leaves it at that.
In the last act, a dead Simon reveals how negative he still is about life. He says that people, in their ignorance, trample on the feelings of others. Simon has been scarred by life and the outward sign of his pain was the drinking he did while he tried to escape his tortured existence. His final escape was suicide.
Near the end of Act I, Dr. Gibbs says about Simon Stimson: "Some people ain't made for small town life." As the organist and director of the church choir, Simon Stimson likely feels rather confined and unstimulated living in small Grover's Corners. As Dr. Gibbs suggests, his drinking is likely the result of his lack of creative stimulation and his boredom with life in a small town. Earlier in Act I Mrs. Gibbs alludes to Simon Stimson's "problems" and we learn that his drinking has gone on for years and has recently appeared to get worse. Although we never learn the details of his "problems," we can only assume that his drinking is the result of his feeling trapped.