It’s this.
To me, saying “this too shall pass” reminds me that life is a journey of good things and bad things my purpose is to experience them all as fully as I can.
Yesterday was great because x, today is shit because y, tomorrow will be new and different because z.
When I’m obsessing and disparing about y, saying “this too shall pass” reminds me that there’s a bigger picture, and that y situation will change just as x did.