Tapasya means voluntarily accepting the rulings of scriptures, spiritual master, saintly person, and mold your life in that way. So He is instructing His sons, "My dear sons, don't spoil your life living like cats and dogs and hogs. Utilize your life by tapasya, by voluntarily accepting the rulings of śāstra, spiritual master, saintly person." The question may be "Why this injunction? Why I shall not live like an animal? And why I have to live under the regulative principles of scriptures and saintly person and spiritual master?" The answer is also there: tapo divyam.
Tapo divyaṁ putrakā (SB 5.5.1), "My dear sons, this form of life should be utilized for tapasya." Why? Yena sattvaṁ śuddhyet: "If you accept these principles of life, then your existential condition will be purified." At the present moment we are contaminated by the material nature, modes, mostly ignorance and passion. So He's advising that if you abide by the rulings of tapasya, then your existential condition will be purified. Yena sattvaṁ śuddhyet.
Then the question may be, "What is the use of purifying?" The answer is also there: yena sattvaṁ śuddhyed yasmād brahma-saukhyaṁ tv anantam (SB 5.5.1). "And when your existential condition will be purified, then you will be situated on the transcendental platform of blissful life." And one may question, "What is that?" . . . na brahma-saukhyam anantam. You are hankering after happiness, pleasure. So when your existential condition will be purified and you will be placed in the transcendental platform, at that time you will enjoy eternal happiness.
You are all . . . after all, you are after happiness. Why you are struggling so much hard in this material existence? For happiness. Why you are after sense gratification? For happiness. Why you want to possess? For happiness. Why you want to become beautiful? For happiness. Why you want to eat so many things? For happiness. You go on. The happiness, your ultimate goal.