Environment.

It is true, we are but faint hearted crusaders, even the walkers, now-a-days, who undertake no persevering never ending enterprises. Our expeditions are but tours and come round again at evening to the old hearth side from which we set out. Half the walk is but retracing our steps. We should go forth on the shortest walk, perchance, in the spirit of undying adventure, never to return; prepared to send back our embalmed hearts only, as relics to our desolate kingdoms. If you are ready to leave father and mother, and brother and sister, and wife and child and friends, and never see them again; if you have paid your debts, and made your will, and settled all your affairs, and are a free man; then you are ready for a walk.

Thoreau - Walking