The fire named Shiva is ever engaged in worshipping the force (of nature). As he always relieves the sufferings of creatures, he is called Shiva.
अग्नियश्च शिवो नाम शक्तिपूजापरश्च सः। दुःखार्तानां च सर्वेषां शिवकृत् सततं शिवः॥