R. Elazar ben Shamua attributed his long life to 3 things, one of them being that he never went up to duchan without making a bracha. Rashi implies that this refers to the birchas hamitzvah - לברך את עמו ישראל באהבה
Why is this considered something special, it is a standard birchas hamitzvah?
Also, why were chazal mesaken the language of "b'ahava"? [Maharatz chiyus says it means kavanas ha'leiv - not so clear what he means. Magen Avrohom says that without love the bracha won't work].
Also, in the gemara in Shabbos R. Yossi comments that he never violated the request of his friends and that even though he wasn't a kohein, when they asked him to go up to duchan he would go. Tosafos in the name of the R"I comments, what is the big deal, what issur is there for a non-kohein to go up [this is very difficult because the pashtus is that it is a violation of an aseh like the rama says (128:1)? 1. Rama says perhaps there wouldn't be an issur if done with kohanim, but most achronim disgaree (m.b.). 2. Another approach brought by the m.b. (3) is that if it is not done b'kavana l'sheim mitzvah, rather just to fulfill the pushing of his friends it is permitted. 3. Maharsha says he went up but did not say anything. 4. Mitzpeh Eisan in Shabbos suggests it is only an issur in the mikdash to use the sheim hameforash], except for that it may be a bracha l'vatala since only kohanim have the mitzvah. Tosafos is very strange. What do they mean that it would only be a bracha l'vatala? It would certainly be a bracha l'vatala, and furthermore a bracha l'vatala is a serious thing! [Maharsha is madchik to explain that he certainly would not have said anything since that would be a bracha l'vatala, and since he didn't say anything there wouldn't be anything wrong with going up].
I would like to suggest an approach that would answer everything. When we look at the nusach of the bracha, it is not a standard birchas hamitzva of אשר קדשנו במצותיו וצוונו rather it is אשר קדשנו בקדושתו של אהרן וצונו לברך וכו. The Radal (back of gemara) points out that perhaps chazal were never really mesaken a birchas hamitzvah for birchas kohanim, just as they weren't for birchas hamazon or kiddush since the whole mitzvah IS a bracha [See Ma'aseh Nissim hagaddah, based on this we don't make a bracha on sippur yetzias mitzrayim because chazal instituted to do the mitzvah in the nusach of a bracha and therefore didn't institute a birchas hamitzva]. Instead chazal were mesaken an option to make a birchas ha'shevach for giving special kedusha to the descendants of Aharon - אשר קדשנו בקדושתו של אהרן וצונו לברך וכו - meaning, they give praise to hashem for giving them kedusha and commanding them that when they give the bracha to klal yisroel it must be done out of love. The entire statement is a shevach to Hashem rather than a birchas hamitzvah. This explains why R' Elazar ben Shamua took pride in making it, because it wasn't required like a standar birchas hamitzvah. It also explains why they stuck in "b'ahava" to turn it into a birchas hashevach. It also explains why Tosafos in Shabbos wasn't convinced that it would be a violation of a bracha l'vatala since even yisroelim may be able to give this shevach.