The reason we have to pass through Purgatory after death is that we have committed sins and have not made satisfaction for them. These are the sins that we have expressed sorrow for, mortal sins which have been confessed and forgiven, but have not been made up for. This is the meaning of  "satisfaction."

Every individual sin must be expiated (made up for) in this life or the next! Not even the slightest shadow of sin or evil can enter the all-holy presence of God.

The graver, the more frequent the sins, the longer will be the period of expiation and the more intense the pain.

It is not God's fault, nor God's wish, that we go to Purgatory! The fault is all our own. We have sinned and have not made satisfaction. Even after our sin, God, in His infinite goodness, places at our disposal many easy and efficacious means by which we may considerably lessen our term of expiation, or even entirely cancel it.

Most Christians, with incomprehensible rashness, neglect these means and so have to pay their debts in the dreadful prison house of Purgatory.

Source Father Paul O'Sullivan, "How to avoid Purgatory."