Bhashya
सत्यं कामादिपूर्वकं पुण्यापुण्ये शरीरग्रहणकारणम्; तथापि कामप्रयुक्तो हि पुरुषः पुण्यापुण्ये कर्मणी उपचिनोति ; काम-प्रहाणे तु कर्म विद्यमानमपि पुण्यापुण्योपचयकरं न भवति; उपचिते अपि पुण्यापुण्ये कर्मणी कामशून्ये फलारम्भके न भवतः ; तस्मात् काम एव संसारस्य मूलम् ।तथा चोक्तमा-थर्वणे- 'कामान्यः कामयते मन्यमानः स कामभिर्जा-यते तत्र तत्र' इति ।तस्मात् काममय एवायं पुरुषः, यत्अन्यमयत्वं तत् अकारणं विद्यमानमपि- इत्यतः अवधारयति 'काममय एव' इति ।यस्मात् स च काममयः सन् यादृशेन कामेन यथाकामो भवति, तत्क्रतुर्भवति- स काम ईषदभिलाषमात्रेणाभिव्यक्तो यस्मिन्विषये भवति, सः अविहन्यमानः स्फुटीभवन् क्रतुत्वमापद्यते ; क्रतुर्नाम अध्यवसायः निश्चयः, यदनन्तरा क्रिया प्रवर्तते ।यत्क्रतुर्भवति- यादृक्कामकार्येण क्रतुना यथारूपः क्रतुः अस्य सोऽयं यत्क्रतुः भवति- तत्कर्म कुरुते- यद्विषयः क्रतुः, तत्फलनिर्वृत्तये यत् योग्यं कर्म, तत् कुरुते निर्वर्तयति ।यत् कर्म कुरुते, तत् अभिसंपद्यते- तदीयं फलमभिसंपद्यते ।तस्मात् सर्वमयत्वे अस्य संसारित्वे च काम एव हेतुरिति ॥
In plain words
Yes, it's true that merit and demerit, and everything desire stirs up before them, are what land a person in a body. But look closer: it's only a person driven by desire who actually piles up merit and demerit in the first place. Once desire is gone, an action can still happen, but it no longer builds up any merit or demerit at all. And even merit and demerit already banked don't ripen into fruit for someone who has no desire left — nothing sets them in motion. So desire, and desire alone, is the root of the whole wandering existence. Scripture says exactly this in the Ātharvaṇa: 'Whoever keeps wanting the things he desires, dwelling on them, is born again and again wherever those desires carry him.' Which is why this person is made of desire, full stop — whatever else he might seem to be made of is real enough, but it isn't the cause, and that's exactly why the text insists: made of desire, and only that. Since he's made of desire, he becomes whatever that desire shapes him into — that's what 'his resolve' means: a longing that starts as the faintest flicker toward something, and if nothing blocks it, it grows sharp and clear until it hardens into resolve. 'Resolve' just means a settled decision, right after which action follows. 'Whatever his resolve is' means the resolve takes exactly the shape that desire gives it, and 'accordingly he acts' means he does whatever is needed to bring about what that resolve is aiming at. And 'whatever he does, that he gets' simply means he ends up with the fruit of it. So both his being made of everything and his being caught in this wandering existence come down to one thing: desire.
Close to the Sanskrit
True, merit and demerit, preceded by desire and the like, are the cause of taking a body; even so, it is a person impelled by desire who accumulates the two deeds of merit and demerit. But when desire has been abandoned, action, even though it occurs, does not become productive of an increase of merit and demerit; even when merit and demerit have already been accumulated, the two, in one devoid of desire, do not become initiators of their fruit. Therefore desire alone is the root of transmigratory existence. And so it is said in the Ātharvaṇa: 'He who, dwelling on them in thought, desires desires, is born through those desires, again and again, there and there.' Therefore this person is made of desire alone; whatever 'being made of other things' there is, that, even though it exists, is not the cause — for this reason the text restricts with 'made of desire alone.' Since he, being made of desire, becomes exactly what a given desire is, he 'becomes that as his resolve': that desire, manifested by the merest slight longing toward whatever object it occurs, if unobstructed, growing fully evident, attains the state of a resolve. 'Resolve' means determination, certainty, immediately after which action proceeds. 'Whatever his resolve is': whatever form the resolve proper to that desire's effect takes, that is his resolve; '[according to] that he performs action': whatever action is fitting to accomplish the fruit of the resolve's object, that he performs, carries out. 'Whatever action he performs, that he attains': he attains the fruit belonging to it. Therefore, in this one's being made of everything and in his state of being a transmigrator, desire alone is the cause.