...is each posture has meaning.
Standing is a sign of reverence. Everyone stands when a judge walks in
the courtroom or when the President walks into Congress. Catholics
stand when the priest walks in, when the Gospel is read, to profess the
creed, etc. We stand in reverence.
Sitting is a sign of learning/reception. We sit in school while
the teacher often stands. We sit for the first two readings to learn.
We sit when the gifts are brought up to be receptive to the gift God is
preparing for us.
Kneeling is a sign of humility and praise.
We kneel out of humility during the Eucharistic prayer when the bread
and wine become the body and blood, soul and divinity of Jesus. We
kneel after receiving the Eucharist in praise our Lord for entering our
lives in such an incredible way.
This is yet another reason why Catholics sit, stand, and kneel so much during the Mass.