The key thing here is that perfection is not the same as “sinlessness.” If it were, none of us would attain perfection because we all inherit from Adam a tendency to sin.
So what is “perfection”? Paul says that the bond of perfection is charity. (Col 3:14) When you love someone you want to be united with them. In the case of our relationship with God, we should of course seek to be united to Him in prayer and in Holy Communion, but we should also seek a union of will with God, which is critical to spiritual perfection.
Take an example. Suppose two women work in an office and both are struggling to deal with a guy who is constantly criticizing and belittling everyone. The first woman says, “I know God wants me to have patience here, but there’s only so much a person can take. If he starts attacking me today, I’m really going to give him a piece of my mind.” The second woman says, “I know God wants me to have patience here, and that’s just what I’m going to do.” Later the first woman responds to a verbal attack by lashing out at her brother. The second woman, when attacked, tries to bear it but because of human weakness has a fall and loses her patience. Both are guilty of a sin in the uncharitable words spoken to their brother but only the first has shown a lack of perfection. It was demonstrated when she formed the intention not to do God’s will.
In fact, although it sounds a little strange to say it this way, there can be varying levels of perfection. One person might form the intention to eliminate all sin from his life. But another might try for an even higher level of perfection: to seek to please God in all things, even when there is no question of sin. For example, it’s not a sin to defend yourself when wrongfully accused. But it’s very pleasing to God when, in imitation of Jesus, you remain silent in the face of a false accusation. In The Story of a Soul, St. Thérèse of Lisieux says that she was one day wrongly blamed for a broken jar that had been carelessly left about by another nun. She didn’t answer when scolded but simply promised to be more observant.
If you go far enough in spiritual perfection, you get to the point where you no longer complain about anything. You know that everything that happens in your life is orchestrated by God and permitted by Him–every illness, financial setback, insult hurled your way, etc. All God’s actions are done from a motive of love and mercy and He allows nothing to happen without some good reason. So why complain? Thank Him, bless Him and exalt his Holy Name instead.