If you say that an idea, emotion, or other factor is the mainspring of something, you mean that it is the most important reason or motive for that thing. 主要部分
noun [抽象名词]主要动力;(钟表)主发条;主要原因;主要动机
My life has been music, and a constant search for it has been the mainspring of my life. 我为音乐而生,对音乐的不懈探索已经成为我生活的动力。