nasso

The Posuk says that 'ה tells Moshe to tell Aaron כה תברכו את בני ישראל; so shall you bless the בני ישראל.
The Medrish says the people said to Hashem אין אנו צריכים אלא לברכתך "we don't need anything, only Your beracha & to be blessed by You. 'ה answers them, even though I told the Kohanim to bless you, I will stand with them & bless you.
The כתב סופר asks about the לשון they used. It doesn't make sense to say we don't need anything except your beracha. That implies that they're settling for His beracha, when in reality, the beracha of 'ה is far greater & more prestigious than the beracha of the Kohen. They should have said אין אנו רוצים we only want your beracha.
Someone once came to the Chafetz Chaim complaining bitterly about his lot. It came out in the conversation that the man had been able to provide for his family but with great struggle & that's not what he wanted. He wanted to be wealthy, so he bought a lottery ticket & prayed day & night that he should win. 'ה answered his prayer & he won. A short time later he lost it all & couldn't even provide food for his family. The Chafetz Chaim said, what can I do for you? He quoted the sefer chesed l'avraham that brings down a chazal that says there's an announcement about each person saying whether he'll be wealthy or poor, & also how much he will make each week to support his family. The Chafetz Chaim continued, if you had parnassa, even if it was tough, why would you trouble 'ה to make you win? Now you got years & years of your money up front, so how could I help you?
Similarly says the כתב סופר, when a person blesses his friend, he blesses him with wealth, long life, honor etc. In truth, he has no idea if what he's blessing him with is what's good for him. Even the person himself doesn't know what's good for him, which is why the Chafetz Chaim says when one asks 'ה for things he should always add "only if it's for my benefit".
This is what the בני ישראל were saying to 'ה. You commanded the Kohanim to bless us but they're going to bless us the way one blesses his friend; not knowing if what they're wishing on him is good or bad. We only need what you know to be a beracha. That's all we want - no more. That's why 'ה said He'll be standing there & blessing along with them, to ensure that the beracha that each person gets is what he needs and what is right for him!