Those who are resolute in this path (of Karma Yoga) have one pointed mind. Many-branched and endless are the thoughts of the irresolute.

— Bhagavad Gita 2.41

You feed your energies to whatever you pay attention to. A karma yogi knows this and s/he does not dissipate his/her energies. A karma yogi takes up a worthy purpose, puts all his/her attention into the purpose and gets it done.

Even physical science has plenty of examples to illustrate the power of focus and attention. When a paper is exposed to sunlight, nothing happens. But when the sun’s rays are focused using a lens into a single point of the paper, so much energy goes into the point that it burns. A piece of iron as such is nothing. But when the polarity of particles in the iron piece is aligned, it becomes a magnet.

We know that the objects of pleasure and pain vary from person to person and time to time, ultimately pleasure and pain have no reality, these are just notions of mind created by habitual responses of the mind and associations. But when we pay too much attention to the thoughts that try to convince us that the pleasure and pain are real, the reality of pleasure and pain grows in our life. When pursuit of pleasure and denial of pain becomes the dominant reality of one’s life, it’s one miserable life. A karma yogi knows this and so is focused only on the purpose at hand, he/she is not infatuated by the notions of pleasure and pain.

Swami Vivekananda says,

Take up one idea. Make that one idea our life — think of it, dream of it, live on that idea. Let the brain, muscles, nerves, every part of your body, be full of that idea, and just leave every other idea alone. This is the way to success.

See also: Karma Yoga (Bhagavad Gita 2.39,40) and Pleasure and pain, heat and cold (Bhagavad Gita 2.14, 15)