You are carried away to external objects through attachment and infatuated love. This is concentration. Concentration or Dharana is centering the mind on one single thought. Vedantins try to fix the mind on the Atman. This is their Dharana.
Concentration is a great necessity for all aspirants.
During concentration, the various rays of the mind are collected and focussed on the object of concentration. There will be no tossing of the mind. One idea occupies the mind. The whole energy of the mind is concentrated on that one idea. The senses become still.