We offer ourselves in the Mass
In the offertory, the people participate very often by bringing up bread and wine. The bread and wine represent the priest and the people. There has to be some symbol by which we all can stand at the altar, because we are offering ourselves to God. We're not attending a prayer. This is a sacrifice. So the good Lord has chosen something that does symbolize very well our life. Namely, bread gives us substance and wine is the very marrow of the earth, it gives us blood. When, therefore, we bring that which gives us our nourishment, we are equivalently bringing ourselves. We are on the altar. But we are not just there as offerers. The wheat had to be ground. The grapes had to be crushed. This is already an intimation of what is going to happen during the Eucharist.