LMA- Let me tell you a vort from the Vilna goan. The passuk says by the mitzva of kibud av and by the mitzva of shiluach hakein lmaan yaarichun yamecha- lmaan yitov lach vchulu. Frekt the Vilna Goan why davka by these two mitzvos does it say the schar? THe Vilna goan explains from a yerushalmi . The yerushalmi explains that by shliach hakein after chasing away the mother she comes back and sees that her kids are missing, the mother has so much tzaar that she goes and commits suicide. ( And therefore one who says al kan tzipor is a to’eh). So really this is a very cruel mitzvah. Kibud av Vaim, on the other hand, is the most sichliusdikke mitzvah out there . The Gra says when one comes to the madreiga that he is able to do both of these mitzvos the same way than we say lman yaarichun. BEcause he understands that he is doing avodas hashem and is able to remove his emotions. Basically, the oan continues and says how you must do a mitzva even if it is cruel because we do avodas hashem for no other reason etc.
HE also uses this to explain why only after the akeida was Avrohom Avinu called yirei Elokim. Because Avrohom Avinu spent his entire life preaching against avodah Zara like the molech where you are makriv a kid. ANd after 138 years of preaching he is told by HAshem to be makriv his own kid, in theory that is the bigest stira to him. But he asked no questions and did not think about how it made no sense , therefore no he is called a yarei hashem.