Canto 2 -
Ayodhyā-kāṇḍa
Chapter 21: Rāma Consoles Kausalyā and Lakṣmaṇa
Text 2.21.42

धर्मो हि परमो लोके धर्मे सत्यं प्रतिष्ठितम्।
धर्मसंश्रितमेतच्च पितुर्वचनमुत्तमम्॥

dharmo hi paramo loke dharme satyaṁ pratiṣṭhitam
dharma-saṁśritam etac ca pitur vacanam uttamam

dharmaḥ hi = Dharma; paramaḥ = is the first and foremost human goal; loke = in this world; dharme = on dharma; satyam = [and] truthfulness; pratiṣṭhitam = rests; dharma-saṁśritam = is fully in line with dharma; etat = the; ca = and; pituḥ = of My father; vacanam = instruction; uttamam = so is superior [to My mother’s instruction].

Dharma is the first and foremost human goal in this world [and] truthfulness rests on dharma. The instruction of My father is fully in line with dharma and so is superior [to My mother’s instruction].

Dharma is the chief among the worldly puruṣārthas. Truthfulness rests on dharma, that is, it culminates in dharma alone.1 This instruction of Śrī Rāma’s father was fully in line with dharma, and it would only lead to pious credits. 

NOTE. Three are the human goals (puruṣārthas) in this world—dharma, artha and kāma. When one earns pious credits, in his next life he is blessed with artha and kāma in the form of good birth (janma), opulence (aiśvarya), education and training (śruta) and beauty (śrī), etc. Therefore, dharma is superior to artha and kāma, and hence it is the chief of these three worldly human goals. In material existence, those who always take shelter of dharma are always placed in positions superior to those who always take shelter of artha and/or kāma later in time, typically in their next lives.

From the technical point of view, the exact injunction of Daśaratha was in line with the Vedic principle of truthfulness—truthfulness to his wife Kaikeyī. And it did not contradict any principle of the varṇāśrama system—Daśaratha did not order Rāmacandra  to engage in any activity forbidden in the scriptures. Daśaratha’s motive behind his instruction to Rāma was undoubtedly defective. But Lord Rāma, as the ideal son, considered that from the technical point of view, Daśaratha’s exact injunction was in accordance with dharma and did not contradict the varṇāśrama system. He disregarded all other aspects of this issue, quite unlike Lakṣmaṇa and Kausalyā-devī.

Why is Lord Rāma so interested in advocating Vedic dharma? Because He appeared to establish it. And why is that? Prabhupāda explains it thus:

Principles of religion are laid down in the Vedas, and any discrepancy in the matter of properly executing the rules of the Vedas makes one irreligious. In the Bhāgavatam it is stated that such principles are the laws of the Lord. Only the Lord can manufacture a system of religion. The Vedas are also accepted as originally spoken by the Lord Himself to Brahmā, from within his heart. Therefore, the principles of dharma, or religion, are the direct orders of the Supreme Personality of Godhead (dharmam tu sākṣād bhagavat-praṇītam).1 These principles are clearly indicated throughout the Bhagavad-gītā. The purpose of the Vedas is to establish such principles under the order of the Supreme Lord, and the Lord directly orders, at the end of the Gītā, that the highest principle of religion is to surrender unto Him only, and nothing more. The Vedic principles push one towards complete surrender unto Him; and whenever such principles are disturbed by the demoniac, the Lord appears. (Bhagavad-gītā 4.7 purport) 

1 Truthfulness culminates in the acquisition of piety.

1 Śrīmad-Bhāgavatam 6.3.19.