Because as spirit soul, we are healthy. Ānandamayo 'bhyāsāt (Vedānta-sūtra 1.1.12). By nature we are joyful. There is no question of our being morose, unhappy, diseased. No. Spiritual life, spirit as it is, it is . . . the constitution is blissful, full of knowledge and eternal. That is spiritual life, actual, sac-cid-ānanda-vigraha (Bs. 5.1).
So we are not actually, I mean to say, constitutionally, we are unhappy. Our happiness is our life. Ānandamayo 'bhyāsāt (Vedānta-sūtra 1.1.12). In the Vedānta-sūtra, you'll find that this verse, ānandamayo 'bhyāsāt. As God is full of joy, similarly, we are also part and parcel of God; we are also full of joy. So we have to treat ourself how to go that platform of joyfulness.
That is explained in the Śrīmad Bhagavad-gītā: brahma-bhūtaḥ prasannātmā na śocati na kāṅkṣati (BG 18.54). You can simply become joyful, without any anxiety. When? Brahma-bhūtaḥ: when you understand yourself, when you understand your spiritual existence. That is Brahman . . . Prahlāda Mahārāja analytically, self-analysis.
So that spiritual existence, Prahlāda Mahārāja, he is giving hint here that, "We have got three stages of our existence. Sometimes we are active, sometimes we are sleeping and sometimes we are deeply sleeping. But within these three stages of our existence, I am there." When I am active, I am there. When I am sleeping, I am dreaming, I am there. And when I am in deep sleep, I am there. And when again I am awakened, I am there. In no case I am not there. I am there always.