TT #165 · Parashas Shoftim · Fourth Aliyah · Passuk 18:06
And if a Levi comes from one of your gates, from all of Yisrael where he sojourns, and he comes with all the desire of his soul to the place that the Eternal will choose,
Torah Temimah points to Sifrei Devarim 168.
TT lemma: וְכִי יָבֹא הַלֵּוִי.
English: ‘And if a Levi comes’ – one might have thought against his will; therefore the Torah says ‘with all the desire of his soul.’
Derash on the voluntary nature (Sifrei) — the phrase ‘desire of his soul’ teaches that the service is undertaken willingly.
A Levi who wishes to come and serve at the chosen place may do so of his own free desire; the service is not forced.