Most of us know the Reform movement’s HaMotzi which begins “we give thanks to God for bread” and concludes with the traditional b’rachah in Hebrew and the word “amen.” I rarely sing the b’rachah itself and prefer to listen to it and then respond with an “amen.” In this way, others have said the blessing on my behalf and I can eat bread without repeating the blessing if I so choose or else I have confirmed their blessing and then can make my own, but without saying “amen.”
This is what I was taught growing up: one doesn’t say “amen” to one’s own b’rachah. Even more, saying amen to your own b’rachah is a sign of ignorance.
Also as a child, I had learned to recite the long form of the Ashkenazi Birkat haMazon / Grace after meals by heart before I could read, since we sang it every Friday evening and Shabbat lunch in my home. In the middle of the Birkat haMazon, I knew that the blessing for rebuilding Jerusalem concluded with: “Blessed is God who rebuilds Jerusalem; Amen.” Naturally, when I was old enough to be in school, I wanted to know why it was permissible to answer one’s own blessing with amen in this case. I was told that originally this was end of the Birkat haMazon and so, in this case, it was permissible.