All mitzvos should preferably be performed in a state of joy. If a person can not attain this emotion, the mitzva must still be performed. The mitzva of birchas kohanim is unique in that if a kohen is not happy, he does not perform this mitzva. It is because of this requirement of joy that a kohen who is in aveilus leaves during birchas kohanim. There are some opinions that a kohen who is not married should not perform the mitzva of birchas kohainim because he also lacks the full degree of happiness necessary for the proper fulfillment of this mitzva. For this reason, ashkenazi communities outside of Eretz Yisroel only perform birchas kohainim on yom tov because we are too occupied with our mundane business to be in the state of mind befitting birchas kohainim. (for details concerning these practices see Shulchan Aruch and Rama, Orach Chaim 128:43-44). Why should birchas kohainim be singled out from other mitzvos to require joy as a prerequisite to its proper fulfillment?
In Michah (5:8), we are taught that Hashem wants us to practice justice and to love kindness. The Chofetz Chaim observes that the precise wording of the pasuk is in contrast to our mindset as we pursue these two lofty goals. Whereas justice has to be carries out, though not necessarily with love, kindness cannot merely be done without love. Giving to others is not a perfunctory act but rather an expression of a will for the well being of the recipient.
Concerning the mitzva of tzedakah we are given a unique prohibition. In Devarim (15:10) we are commanded not to resent giving tzedakah. Physically, giving tzedakah while resenting the act disqualifies it as a fulfillment of ahavas chessed. The proper frame of mind is critical for fulfilling these mitzvos because of the unique opportunity they provide us which is to imitate Hashem in all His ways. Just as Hashem not only performs acts of kindness, but is One whose essence is goodness, so too our personalities are supposed to become synonymous with goodness. An act of chessed or tzedakah done without the acknowledgement that such acts are a privilege can never transform individuals into those whose very personalities are defined by kindness.
The kohen who is called upon to bless the people is not asked to merely pronounce the words. Birchas Kohanim is not like other verbal mitzvos that can technically be fulfilled regardless of one's mood. The kohen is called upon to echo the blessing of Hashem unto His people. To bestow a blessing is the ultimate kindness. Just as Hashem blesses us as an expression of His kindness, so to the kohen must emulate Hashem in this manner. A blessing must be accompanied by joy or else it is the equivalent of tzadaka given with resentment. If a kohen cannot reach this level it is better for him to leave shul rather than bless half-heartedly. If kohanim cannot properly bless Hashem's people all year long, it is preferable to delay the bracha until yom tov when it can be given properly.
The bracha recited before birchas kohanim is "l'varech es amo Yisroel b'ahava" ("to bless His nation Israel with love"). Although the phrase "with love" is not recited in the berachos preceding other mitzvos, love and joy are integral to this mitzva. May we merit to see the brachos of birchas kohanim fulfilled and soon see this mitzva be performed in the Beis Hamikdosh as Hashem truly will bless us with His love.