His love for music was the mainspring behind his becoming a composer.

mainspring 由 main（主要的）和 spring（弹簧、发条）组成，字面意思是"主发条"，指机械钟表中提供动力的主要弹簧。在这里用作比喻意义，表示"主要动力"或"关键推动力"。