Stones has ousted Aymeric Laporte from Guardiola's first-choice defence after making just 16 Premier League appearances last season.
The centre-back, signed for £47 million from Everton in 2016 has benefitted from staying injury free but Guardiola has also put Stones' good form down to "a settled personal life" after splitting from his long-term girlfriend in November 2018.
"His attitude was always the same but we see him happier and stronger now because he's fit, he's not injured every two or three weeks, and that helps a lot," Guardiola told a news conference on Friday.
"He's played really well and that gives you confidence. His personal life, now it's settled, is much better. He's a guy who is so sensible and that helps to be focused on what we have to do.
"My only advice to him is that what he's gained right now, in the way he lives his life and how he takes care of himself -- his body, his mind, everything -- he's got to keep that for the next 10 years, 12 years, here with me, without me, with other managers.
"The moment that drops he will not be consistent."
Stones was one of the first players through the door at the Etihad Stadium following Guardiola's arrival in 2016 but has only once managed more than 40 games in a season during his four-and-a-half years at the club.
"If he can avoid injuries he can be the player we've always believed he can be," said Guardiola. "He's given us a good feeling but it's just two or three games. A defender has to be consistent all season."