Sive was born in Brooklyn, New York, on September 22, 1922, the son of Abraham Sive and Rebecca (née Schwartz) Sive. As a teenager, his growing love for the outdoors and fascination with the American wilderness, as well as his interest in the writings of Thoreau, Emerson, and Wordsworth, led him to a lifelong passion for the natural environment, to wilderness preservation and environmental protection. Hiking and camping expeditions during his college years, to the Catskill and Adirondack mountains of New York State, foreshadowed his advocacy in later years for the “forever wild” clause in the New York State constitution and his activism for environmental preservation in his home state and throughout the U.S.