The most obvious cause of difficulty in selectively attending to one's studies is distraction by automatic attention. If other people in a classroom are making a commotion, it is hard to listen carefully to a lecture, or if a neighbor is playing loud music, it is hard to concentrate on a textbook. As matters have it, such problems are not the major ones. The disturbing events are usually only temporary and if they persist, there is an adaptation process that makes them seem less powerful. Just as your body gets used to the water after you have been in a swimming pool for a while, so too does your mind get used to background noises. By-and-large, people can maintain selective attention in spite of competition from automatic attention processes.
However, there is a limit and there is a price. The limit is simply that you cannot completely adapt to very potent stimuli. Returning to the swimming pool analogy, if the water is very cold, you will not get used to it. Similarly, you cannot completely ignore a neighbor's music that is very loud. The price is that it takes more mental effort to attend to one's studies in a distracting environment. Even your own music playing softly will make you feel tired studying sooner. Selective attention is an effortful response.
The major challenge for the serious student is to keep selective attention focused on study materials rather than on day-dreams, personal problems, or social activities. Although attention is a covert activity, our best hypothesis is that it obeys the same principles that have been discovered by research on overt behavior. One of these principles is that of minimizing work (least effort):
Attending to a difficult lesson is hard work while attending to enticing daydreams is easy. Even worrying about personal problems is relatively easy because you have probably practiced worrying about them a great deal in the past. In effect, these various types of thoughts are competing for your selective attention and the Principle of Minimizing Work implies that your attention will naturally have a tendency to be diverted into the easier response. This tendency for your "mind to wander," even without any external distractions, is inevitable and you need to know how to recognize when this is happening and how to get your mind "back on track."
I believe that the ability to focus and sustain attention on relevant material is the most important trait of a good student. To be sure, an adequate level of intelligence is necessary, as is a full measure of motivation, but selective control of attention is what most separates the good from the poor student. This belief is not original with me. Sir Isaac Newton, whom many consider to be the father of modern science, said, "If I have ever made any valuable discoveries, it has been owing more to paying attention than to any other talent."