My first gallery show was all landscapey-type photos. A photographer friend came up to me during the opening and said "Where are all the people?" That comment stuck with me and led to doing nudes and portraits.
Another inspirational comment came from my wife, who some years ago told me never to use her camera (a point&shoot) again, as I just wasted film with the terrible pictures I took. Which led to my getting a "real" camera and the determination to show her I could take decent photos. She eventually conceded that I did indeed have some talent, except for some of the nudes, which she considers indecent.