N-COUNT
（观点、情感等的）主要原因，主要动机
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.
例句
My life has been music, and a constant search for it has been the mainspring of my life...
我以音乐为生，对音乐的不懈探索已经成为我生活的动力。
You begin to understand what actions were the mainspring of the story.
你开始体会到是什么样的行动在推动这个故事的发展。