Convention, OK, this negative sign is just a pure convention. You just got to use it all the time. If you use an old textbook, written when I was taking thermodynamics, they have the opposite convention, and it's very confusing. But now we've all agreed on this convention, and work is going to be with the negative sign here.

系统对外界做功的符号为负,这就是我们,在本课中的规定，纯粹出于统一符号的需要，有些教科书中，可能会使用相反的符号规定,注意不要混淆了，这门课上,我们将一直使用。

麻省理工公开课 - 热力学与动力学课程节选