SB 3.6.33

padbhyāṁ bhagavato jajñe
 śuśrūṣā dharma-siddhaye
tasyāṁ jātaḥ purā śūdro
 yad-vṛttyā tuṣyate hariḥ
Synonyms: 
padbhyām — from the legs; bhagavataḥ — of the Personality of Godhead; jajñe — became manifested; śuśrūṣā — service; dharma — occupational duty; siddhaye — for the matter of; tasyām — in that; jātaḥ — being generated; purā — formerly; śūdraḥ — the servitors; yat-vṛttyā — the occupation by which; tuṣyate — becomes satisfied; hariḥ — the Supreme Personality of Godhead.
Translation: 
Thereafter, service was manifested from the legs of the Personality of Godhead for the sake of perfecting the religious function. Situated on the legs are the śūdras, who satisfy the Lord by service.
Purport: 

Service is the real constitutional occupation of all living entities. The living entities are meant to render service to the Lord, and they can attain religious perfection by this service attitude. One cannot attain religious perfection simply by speculating to attain theoretical knowledge. The jñānī division of spiritualists go on speculating only to distinguish the soul from matter, but they have no information of the activities of the soul after being liberated by knowledge. It is said that persons who only mentally speculate to know things as they are and who do not engage in the transcendental loving service of the Lord are simply wasting their time.

It is clearly said here that the principle of service was generated from the legs of the Lord for the sake of perfecting the religious process, but this transcendental service is different from the idea of service in the material world. In the material world, no one wants to be a servant; everyone wants to become the master because false mastership is the basic disease of the conditioned soul. The conditioned soul in the material world wants to lord it over others. Illusioned by the external energy of the Lord, he is forced to become a servant of the material world. That is the real position of the conditioned soul. The last snare of the illusory, external energy is the conception of becoming one with the Lord, and due to this conception the illusioned soul remains in the bondage of material energy, falsely thinking himself a liberated soul and “as good as Nārāyaṇa.”

It is actually better to be a śūdra than to be a brāhmaṇa and not develop the service attitude, because that attitude alone satisfies the Lord. Every living being — even if he be a brāhmaṇa by qualification — must take to the transcendental service of the Lord. Both Bhagavad-gītā and the Śrīmad-Bhāgavatam support that this service attitude is the perfection of the living entity. A brāhmaṇa, kṣatriya, vaiśya or śūdra can perfect his occupational duties only by rendering service unto the Lord. A brāhmaṇa is supposed to know this fact due to his perfection in Vedic wisdom. The other sections are supposed to follow the direction of the brāhmaṇa Vaiṣṇava (one who is a brāhmaṇa by qualification and a Vaiṣṇava by action). That will make the entire society perfect in regard to the order of its social construction. A disordered society cannot satisfy either the members of the society or the Lord. Even if one is not a perfect brāhmaṇa, kṣatriya, vaiśya or śūdra but takes to the service of the Lord, not caring for the perfection of his social position, he becomes a perfect human being simply by developing the attitude of service to the Supreme Lord.