Soz means burning — an inner heat, an ache, a warmth of feeling. It is one of Iqbal's most characteristic words, and for him it is a sign of life itself. A self with soz is alive; a self without it, however clever or accomplished, has gone cold.
Iqbal feared coldness more than almost anything — the chill of pure calculation, of a heart that has stopped feeling, of a clever mind with no fire beneath it. He even prayed, in one famous verse, that a storm be sent to a man whose sea had grown too calm. Comfort without soz, to Iqbal, is a quiet death.
The question soz puts to you is simple and searching: is there still heat in what you do, or are you only going through correct motions? Iqbal would rather you struggle warmly than succeed coldly. The fire is not a problem to manage — it is the proof that you are alive.
See it in the verse
Lahu garm rakhne ka hai ek bahana
Ki tere bahr ki maujon mein iztirab nahin
Tera dam garmi-e-mahfil nahin hai