Xin Xianying was essentially implying that Cao Pi should express sadness because his father must die before he can become the next ruler, and that he should feel intimidated by the fact that he would have to shoulder the heavy responsibility of ruling a kingdom.
Xin Xianying indicated that Cao Pi ought to express sadness due to his father's inevitable demise before he can ascend to power, and he should feel apprehensive about the immense responsibility of governing a realm.