Our Gemara on Amud Beis discusses the importance of saying Amen after a blessing:

רַבִּי יוֹסֵי אוֹמֵר: גָּדוֹל הָעוֹנֶה ״אָמֵן״ יוֹתֵר מִן הַמְבָרֵךְ. וְאָמַר לוֹ רַבִּי נְהוֹרַאי: הַשָּׁמַיִם! כָּךְ הוּא: תֵּדַע שֶׁהֲרֵי גּוּלְיָירִים מִתְגָּרִין בַּמִּלְחָמָה, וְגִבּוֹרִים נוֹצְחִין.

Rabbi Yosei says: The one who answers amen is greater than the one who recites the blessing? And Rabbi Nehorai said to him: By Heavens, it is so. Know that this is true, as the military assistants [gulyarim] descend to the battlefield and initiate the war and the mighty follow them and prevail. The amen that follows a blessing is compared to the mighty who join the war after the assistants, illustrating that answering amen is more significant than reciting the initial blessing.

Beis Yosef (Tur OC 215) says that when it comes to answering amen to a thanksgiving blessing over a food or other benefit, as opposed to a blessing over a mitzvah or obligatory prayer, answering amen is optional.  Rav Shlomo Kluger (Haelef Lecha Shelomo 95) asks, since we say that the one who answers amen is greater than one who makes the blessing, how is it possible that he is not obligated to answer amen?  If we were to say, that he is not obligated to answer, but still it is somehow greater, that would not make sense because we have a general principle, “One who is obligated in a mitzvah receives more reward than one who is not and still chooses to do so.”?

Rav Shlomo Kluger answers that the entire principle of one who responds "amen" is greater than the one who says the blessing does not apply to blessings that are for  thanksgiving blessing over a food or other benefits because, by definition, there is no obligation to make such blessings; one can simply choose not to eat.  True, the one who chooses to eat is obligated to make a blessing, but the listener is not,because he or she is not eating.  However, if one hears a blessing over a mitzvah, even if he or she (the listener) is not obligated to make that blessing at that time, such as if that mitzvah was already fulfilled by the listener, there is still an obligation to answer amen because the person is in the category of one who is obligated.

I have some thoughts about Rav Shlomo Kluger’s original objection.  First, his assertion that if amen is not required on a thanksgiving blessing it would be subject to the rule of “One who is obligated in a mitzvah receives more reward than one who is not and still chooses to do so” may not be as straightforward.  It is possible that this principle applies when the person is categorically not obligated, such as a woman performing a mitzvah that is time bound.  But since there is a general obligation to recite thanksgiving blessings on food and other benefits, even if the one listening is not obligated to answer amen, he or she may still receive a greater reward because it is a mitzvah that he or she is generally obligated in. In other words, the entire principle of  “One who is obligated in a mitzvah receives more reward than one who is not”, might be dependent on the idea that there is no circumstance of obligation, thus the person is not related to the mitzvah.  Think about it, if mitzvos are some kind of spiritual medicine, certain medicines only work for certain people.  Thus the wattage or benefit from a mitzvah performed by a person who is not commanded in it, will naturally confer less benefit.  However, in the case of answering amen, even if not obligated, since the person WOULD be obligated to recite a blessing if he WAS eating, it could even bring more benefit to the amen responder than the reciter of the blessing since it is still of benefit to his soul.

Another concern I have with Rav Shlomo Kluger’s argument, is that he is taking the idea that “one who answers amen is greater than the one who recites the blessing” in an absolute sense, across the board.  However, this is not the style of aggadic statements. For example, when it says, “Anyone who humiliates another in public, it is as though he were spilling blood” (Bava Metzia’a 58b), are we to believe such a person is really a murderer?  Halakhically, is one obligated to martyr themselves to avoid transgressing as if it was murder?  One would think not! (Though Tosafos Sotah 10b “Noach” implies yes. However, Meiri Berachos 43b says it is not literal.  We will discuss this more Be”H on Daf 10 Psychology of the Daf Sotah.) So we could simply say that in a certain aspect answering is greater than reciting, perhaps because it shows more honor to Hashem as the Gemara explained in the Mashal of the mighty warriors who follow after the foot soldiers.  

Rav Kuger might argue that even if some aggados are not literal, that is when it uses a qualifier, such in Gemara Bava Metzi’a it says “AS IF spilling blood”, or the Gemara Shabbos 105b that compares breaking vessels in anger to idolatry, it says, “One should CONSIDER it as if…” But here, there is no qualifier, so perhaps Rav Shlomo Kluger’s argument is valid to take it in absolute terms that “one who responds amen is greater than the one who says the blessing.” It does not say, “it is CONSIDERED to be”, but rather, it is.

Still, I believe my first argument, regarding the distinction between not being ever obligated in the mitzvah versus situationally exempt is a stronger point.  If my argument has merit, we still might be able to say that even the amen of a thanksgiving blessing is greater than the reciting the actual blessing.