God created a New Covenant for us, through Jesus Christ. The Eucharistic Prayer (as well as the Apostle's Creed) is the terms of this Covenant. Every time we accept the Covenant through our proclaiming the Great Amen and sealed with partaking of The paschal Lamb, we choose to accept the blessings and curses that come with that Covenant.
Every time I proclaim the Great Amen, whether in voice or song, and eat of the flesh of the Son of Man, the paschal mystery, the Eucharist, I choose to accept the blessings and the curses, based on my adherence to that Covenant.
So, every time I give unselfishly, forgive my neighbor, sacrifice or pray for someone, or love a stranger, I choose to accept the blessings of the Covenant, the blessings that God may grant me for adhering to the terms of the Covenant.
However, every time I sin, or say something against my neighbor, or harbor hatred, or refuse to put God first and foremost in my life, I choose the curses. I choose. God didn't choose for me, I chose! God gave me free will, the freedom to choose between good and evil. If I do not choose God, I choose the curse of my decision.
At one time, I used to be a 'cafeteria Catholic' - Catholic only in name, not in deed. I chose which parts of the Catholic faith I wanted to believe and act upon. Birth control, death penalty (in some cases), and gay marriage were some of the things I used to think the Catholic faith was too old fashioned and needed to come into the 21st century. Because of my actions, talking with friends, or even electing a candidate who believed in these things, I was not choosing God's way - I was choosing man's way. Because I was still receiving the Most Holy Eucharist, I was also choosing the curses that came with my decision.
This brings a very profound realization to mind when I receive the Most Holy Eucharist now. How am I going to behave? What am I going to decide?
Blessing ... or ... Curse