It was just a gradual development over the years. Last year was "All You Need Is Love." This year it's "Give Peace a Chance." Remember love. The only hope for any of us is peace.
Source, in full
Statement to the press (July 1969) after the release of the Plastic Ono Band's single "Give Peace a Chance", as quoted in The Beatles: An Oral History by David Pritchard and Alan Lysaght (1998) New York: Hyperion. ISBN: 0786864362. OCLC: 39093547. p. 285.