आत्माकम्पः सुखात्मा स्फुरति तदपरा त्वन्यथैव स्फुरन्तीस्थैर्यं वा चञ्चलत्वं मनसि परिणतिं याति तत्रत्यमस्मिन् ।चाञ्चल्यं दुःखहेतुर्मनस इदमहो यावदिष्टार्थलब्धि -स्तस्यां यावत्स्थिरत्वं मनसि विषयजं स्यात्सुखं तावदेव ॥ ७३ ॥
In plain words
Ātma, whose own nature is joy, never trembles; Manas, the mind, moves between steadiness and restlessness. That restlessness gives rise to suffering. Pleasure from an object lasts only while the mind holds still in the moment of obtaining what it wanted.
Close to the Sanskrit
Ātma is unmoving and has happiness for its nature; the other principle shines in another way. Stability or restlessness undergoes change in Manas, the mind. Restlessness of mind is the cause of sorrow. Only as long as Manas remains steady upon gaining the desired object does happiness born of that object last.