We are bound by learned habits. We are often on auto-pilot, not conscious of all the sensory information we are receiving, instead reacting from our auto-associative memory faculty, responses acquired through years of repetition.
Attention is a state in which our automatic responses are re-conditioned by awareness of our actions. This occurs naturally through novelties in our environment, or, with greater difficulty, through deliberate conscious awareness of our environment and the activity of our mind.
For the latter, the difficulty arises in maintaining attention. With no novel stimulus in the environment, the mind (i.e. thoughts and feelings, if it can be so divided) will not sit still, will not allow its ramblings to be ignored or permit itself to be reconditioned.
"Yoga is the intentional stopping of the spontaneous fluctuations of the mind".
- Patanjali (~2500 CE)
If one shuts oneself off from sensory stimuli, by sitting rigid and still ("like a tiger ready to pounce"); in silence; eyes closed; steady breath; the only remaining sense is the internal one - the mind. If one then tries to quiet or ignore the activity of the mind, a surprising phenomenon occurs: The mind violently refuses to be ignored, using every one of its powers to bring attention back under its control. Its weapons are many and varied: it invokes old memories of regret, anxiety for the future, feelings of all varieties including boredom, futility, anger, even attempts to break the posture and the rhythm of the breath to return them to activity. Each time it is fended off, it readily attacks from a new side.
The mind wages war to reclaim the faculty of attention. I know of no framework which can adequately justify the reason this is so. Suffice it to say, those who try or have tried this exercise will not doubt the truth of it.
This phenomenon and methods of experiencing and conquering it have fallen under many names. It becomes wrapped in mysticism and superstition, yet is so plain and practical that anyone, even the staunch materialist can experience it and learn many things about himself from it. The less said, the better understood. I can only demystify it, and hopefully inspire you to try the above exercise. Anything more I could say would be pandering to the mind.