When the thought of doing good to others becomes a part and parcel of man's life, of his very being, then he will not entertain the least selfish motive. He will take immense delight in serving others and in doing good to others.
There is a peculiar joy and bliss in vigorous selfless service. You get inner spiritual strength and power by performing motiveless and selfless service.
You will develop humility, pure love, sympathy, tolerance and mercy. The sense of separateness will be annihilated. Selfishness will be eradicated. You will get a broad and liberal outlook on life. You will begin to feel oneness and unity. Eventually you will attain knowledge of the self.
If you really want to grow in the spiritual path, you must do all sorts of service daily, till the end of your life. Then only you are safe. Do not stop serving when you become a famous yogi. The spirit of service must enter every nerve, cell, tissue and bone of your body. Then you will become a real, full-blown practical vedantin.
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This is a prayer for karma yogis by Sri Shankaracharya: "Thou art Atman, buddhi is thy consort Parvati; the prana is thy attendant; this body is thy house; the act of sensual enjoyment is thy worship; deep sleep is samadhi; walking by my feet is the perambulation around thee; all my speeches are thy praise; whatever actions I perform, are all thy worship, O Lord Siva".