When you look up at the sky, why do you sometimes have to look at a star to see another one that is next to it?
Stars have different brightnesses. There's an astronomical brightness scale (see link) that categorizes the brightness of celestial objects. The brightest stars or objects are of course the most easily seen. However, there are many more faint than bright stars, and its not uncommon for several fainter stars to appear near a bright one. If you look at a bright star through binoculars, you will see other fainter (and probably more distant) stars around bright one. Look at the second star of the handle of the Big Dipper some night. If your eyesight's good enough, you may see a fainter star right next to it.
Also, the human eye sees better under low light conditions if its not directly focused on the object you're looking at. The black and white receptor "rod" cells in the eye are more sensitive than the color receptor "cone" cells, so looking at one star may bring a nearby fainter one into your field of vision.