... I remember you and recall you without effort, without exercise of will; that is, by natural impulse, indicated by a sense of duty, or of obligation. And that, I take it, is the only sort of remembering worth the having. When we think of friends, and call their faces out of the shadows, and their voices out of the echoes that faint along the corridors of memory, and do it without knowing why save that we love to do it, we content ourselves that friendship is a Reality, and not a Fancy -- that it is built upon a rock, and not upon the sands that dissolve away with the ebbing tides and carry their monuments with them. 

Samuel Clemens