In Egyptian and other cultures the bull was highly valued. "The Egyptians viewed the bull as the vehicle on which a god rode in power, and as such they identified it as divine itself" (Constable's Commentary).
From Aaron's point of view he was still worshipping the Lord but doing so via means of the calf. This was clearly not what God intended and it created such a rift between God and his people that Moses pitched the Tent of Meeting outside the Israelites camp "some distance away" (Exodus 33:7). God wanted to dwell in the midst of his people but their attitude drove him away.
Aaron's problem began when Moses was a long time on the mountain (v.1). When nothing appeared to be happening Aaron took circumstances into his own hands. We also see this in Saul's life when he offered the sacrifice himself instead of waiting for Samuel (1 Samuel 13:8-14) and in the life of Abraham when he fathered a child through Hagar (Genesis 16:2). Likewise we create problems when we do God's work our way.
Today many worship God according to their timetable, their convenience and their traditions but it does not draw them any closer to him. In fact, it drives God away. God will not be worshipped according to a man made formula.
Later Jesus would say to the woman at the well: "A time is coming and has now come when the true worshipers will worship the Father in the Spirit and in truth, for they are the kind of worshipers the Father seeks." (John 4:23).
God is still seeking those who will worship him in spirit and truth.