The short answer to your question would be that Jesus essentially commanded us “do this in remembrance of me”, and that what we are, as Paul puts it in his first letter to the Corinthians, engaged in a “participation” in the body and blood of the Lord. This is food for the Christian journey. Jesus is the vine and we are the branches. We receive grace and strength to sustain us in our Christian walk through the Eucharist. Jesus stands eternally before the throne and the Father in heaven as a “lamb that has been slain”[Rev 5]. Jesus is eternally offering his death on the cross as expiation for our sins and we participate in that through the sacrifice of the mass and the Eucharist.
The book of Hebrews says that Jesus is a priest forever according to the order of Melchizedek. That implies that as a priest he is offering a sacrifice forever. It just happens to be the “once and for all sacrifice” that he made through his suffering, death, and resurrection. We are allowed access and participation in that eternal sacrifice through the precious body, blood, soul, and divinity of the Lord in the Eucharist.
The following is a very important passage from the book of Hebrews that is tied to what Paul says about our “participation” in the body and blood of the Lord, and it is focused on your question. Hebrews 10:19-27 says:
“Therefore, brethren, since we have confidence to enter the sanctuary by the blood of Jesus, by the new and living way which he opened for us through the curtain, that is, through his flesh, and since we have a great priest over the house of God, let us draw near with a true heart in full assurance of faith, with our hearts sprinkled clean from an evil conscience and our bodies washed with pure water. Let us hold fast the confession of our hope without wavering, for he who promised is faithful; and let us consider how to stir up one another to love and good works, not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near. For if we sin deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful prospect of judgment, and a fury of fire which will consume the adversaries.”
How do we enter the sanctuary that Jesus entered carrying his own blood in sacrifice? We do so through the Eucharist as the representation and “participation” in his body and blood. And notice that with our hearts are sprinkled clean from an evil conscience and our bodies washed with pure water we can “draw near” Also notice that we are not to neglect meeting together because doing so is to sin deliberately and that there will no longer remain a sacrifice for sin on our behalf. There will, instead, be a fearful prospect for judgment. Jesus commanded that we celebrate the Eucharist which is the memorial and re-presentation of his once and for all sacrifice for sins. Neglecting to attend mass and receiving the Eucharist is an incredible rejection of Christ’s redemptive work on the cross, whereas receiving the Eucharist is a “participation” in the body and blood of Christ unto salvation.
I hope this helps and God bless.