Math explanation: When θ = 0, cos θ = 1, so Fd is a maximum. For any other value of θ up to 90 degrees, cos θ < 1 so Fd will be smaller. For θ = 180, you're pushing away from the direction of displacement so you're actually doing negative work.
Intuitive explanation: you have the most success pushing an object when you apply the force directly against it compared to if the force was directed at an angle.
