What we focus upon, we internalize. Focus is our cognitive magnifying glass: the more intense the focus, the deeper our experience.
Most of the time, we do not focus. We move from thought to thought, activity to activity, and internalize very little. We can participate in conversations at a party and retain little of the content. We can flit from thought to thought on a long drive in the car and retain little of the surroundings we pass. This is actually adaptive, as we would quickly become overwhelmed if we internalized all stimuli all the time. When we exercise focus, we select what we will internalize, as in the case of studying for a test or gazing into the eyes of a loved one.
More here – Forbes