Because of the involvement of the senses and the oppression of lust and disease. Where is the happiness in youth when you have not always achieved that?
ईर्ष्यया च महद्दुःखं मोहाद्रक्तस्य तस्य च ।। नेत्रस्य कुपितस्येव त्यागी दुःखाय केवलम् ।। ५३ ।।
Then, he feels misery from malice and rivalry. By delusion his eyes become red like those of an infuriated man. Its abandonment is only conducive to misery.
न रात्रौ विंदते निद्रां कामाग्निपरिवेदितः ।। दिवापि च कुतस्सौख्यमर्थोपार्जनचिंतया ।। ५४ ।।
During nights he does not get proper sleep because he is scorched by the fire of lust. Where is pleasure even during the day because he is worried over the ways and means of earning wealth?