There are various theories about Booth's motivations. In a letter to his mother, he wrote of his desire to avenge the South.
Doris Kearns Goodwin has endorsed the idea that another factor was Booth's rivalry with his well-known older brother, actor
Edwin Booth, who was a loyal Unionist.
David S. Reynolds believes that, though disagreeing with his cause, Booth greatly admired the daring of
abolitionist John Brown; Booth's sister
Asia Booth Clarke quoted him as saying, "John Brown was a man inspired, the grandest character of the century!"