Concentration develops when the intellect is clean and clear.
Expression: For the one whose intellect continues to wander, concentration becomes difficult or impossible. On the other hand, for the one who is focused on one thing there are no other distractions at that time and there is easy concentration.
Experience: When I am able to remain focused, I am not troubled by distractions which lessen my concentration. Instead I am able to increase my discrimination power and am able to decide and judge well. This happens because my intellect becomes clean and gets cleared of waste.
Source:Brahma Kumaris