Q. Why do individuals say Amen after reciting the U’v’nei Yerushalayim (“Rebuild Jerusalem”) blessing in the Grace After Meals? I thought Amen was only meant to be said in response to another person’s blessing.
A. Generally there is no Amen after one’s own b’rachah.
An exception is made at the end of the third paragraph of the Bensching so that even a person saying Grace alone concludes with Amen.
The reason is that the Biblically ordained Grace ended at this point; the fourth blessing and remaining passages were introduced by the rabbis (B’rachot 45b).