From his Playboy interview in 1963:
Sinatra: ... I get an audience involved, personally involved in a song—because I’m involved myself. It’s not something I do deliberately: I can’t help myself. If the song is a lament at the loss of love, I get an ache in my gut. I feel the loss myself and I cry out the loneliness, the hurt and the pain that I feel.
Playboy: Doesn’t any good vocalist “feel” a song? Is there such a difference.…
Sinatra: I don’t know what other singers feel when they articulate lyrics, but being an 18-karat manic-depressive and having lived a life of violent emotional contradictions, I have an overacute capacity for sadness as well as elation. I know what the cat who wrote the song is trying to say. I’ve been there—and back. I guess the audience feels it along with me. They can’t help it. Sentimentality, after all, is an emotion common to all humanity.