The gemara in brachos and halacha in shulchan aruch says that if one say amen after their own bracha it is meguna or not proper, but if one says amen after boneh yerushalayim it is meshubach or praisworhy: Why is this meguna after my own bracha? If it is meguna than why can the same act be all of a sudden praiseworthy? Just because there is a reason to say amen after boneh yerushalyim, why does that make it all of a sudden praiseworthy if one minute ago it was not proper? Why not have a different response to show the end of a seder instead of a response that appears meguna?
It is improper (or meguneh) to answer amen after one’s own blessings. This is not because of how it looks to others, but an intrinsic property of the berachah: the concept of amen is an affirmation, and one cannot affirm one’s own statements. In the words of Eliyah Rabba (167:7, quoting from Behag), one who answers amen after his own blessing is considered to have spoken devarim beteilim — it lacks the proper meaning of amen. However, in Boneh Yerushalayim and other places, Chazal wished that the word amen should be used in expressing the end of a section, and because it does not come only as an affirmation of the immediate blessing, but as a symbol of the ending of the section, it is no longer considered improper.