It might seem as if there would be no particular hurry to photograph a motionless stairwell--except for fact that the light was constantly changing because of clouds moving across the sun. There were also people walking up and down the stairs. If I hadn't taken this image when I did there is no guarantee I could have gotten it any other time.
One of the realities that my trip to Ecuador has reinforced is that there are few second chances in photography. Subjects are often in constant motion. Even when they aren't, the light is changing or the composition is changing. The sun is moving, clouds are moving, people and cars are moving. If the light is changing, so is your exposure. If your subject is moving, your plane of focus may need to change along with it.
Even as all this is happening, you may or may not have a camera with you. If you do, you may be occupied with some other activity, such as answering a call on your cell phone, riding in a cab or a bus, or carrying something in your arms. There is either no time to get your camera ready or by the time you do the moment has passed.
Mind you, I'm not complaining about any of this: I'm simply describing the way things are. It's one of the constant challenges of photography and one of the reasons why we feel such exhilaration when everything falls into place at exactly the right moment. The one thing to remember is that these moments only happen when you're actively photographing; and by "actively," I mean either seizing the moment or waiting for the moment to arrive. Once it comes it may never come again.