When we move on a horizontal surface force of gravity on the center of gravity of our body is perpendicular to the surface and there is no horizontal component to do work on our body.
But when we move on a inclined plane perpendicular to it the force of gravity on the center of gravity is not perpendicular to the surface. It has a component (mgsinA) which tries to do work on our body and will make us fall. But if we make an angle (90-A) with the surface the force of gravity there will be no component which will be able to work on the center of gravity. So we have to lean towards the surface at an angle (90-A). Otherwise we will fall because physical laws make no discrimination. It is same for all.