Why is depth of field affected by focal length?

As the focal length decreases, the depth of field increases as well. Why is this? I'm not so much interested in a physics lesson as I am interested in a simple, down-to-earth explanation.

Answer

Pretty sure I answered this one before but I cannot find it.

• As focal-length gets longer, the angle of view gets smaller.


• With a smaller angle of view, rays forming the image are closer to being parallel.


• With less variation of angle between rays, light has to travel more before being sufficiently out of focus.

This is a little oversimplified but I hope it is easy to visualize at least.