Nice review of The Goldilocks Enigma by Paul Davies by novelist Andrew Crumey:
To understand the enigma of the title, think of the way that stars burn. Stars are made of hot gas held together by gravity, and if the force of gravity were stronger than it is, stars would burn far more quickly and expire sooner. Our own Sun would have had a lifetime of only a few million years instead of the five billion it has already enjoyed, and life on our planet could never have had time to evolve. Equally, if gravity were weaker, then stars would burn too dimly, and their energy output would not support life. It seems that the strength of gravity, like Baby Bear's porridge, is "just right".