Mr. Mu Xin said that any idealism is sentimental. The youth we have had is the frivolous youth we once hoped for, but it is also a regret that will be covered up when we grow up. No one's youth is not idealistic, so no one's youth is not filled with more or less sentimental sentiments. Perhaps, this is the meaning of human existence. Leave in order to come back, remember in order to forget.