It's because God loves us with our sin and brokenness that we know how to love others. Peter denied Jesus three times. Did Jesus make Peter feel small? No, Jesus said nothing to Peter and understood that Peter was a broken man. Peter had sworn that he wouldn't deny Jesus, but he did three times. Our best intentions sometimes aren't good enough. Jesus had a knowing look of disappointment because love hopes for the best, but also recognizes how frail the human condition can be.
When you look at someone else, realize that you are seeing yourself. You aren't unkind to yourself when you recognize your brokenness. Why be unkind to those who are human beings just as you are? Yes, you may be broken in a different way than another person, but you are both broken. And this is where Jesus' words about "do unto others" can help you: others look over your imperfections, so you should look over their imperfections. But you must know God's love in your own life to share it with others!
LaGard says: "So what is the spiritual antidote to our temptation to be unkind? One of the best ways of learning compassion is to appreciate that the weakness we see in others--that same weakness that disappoints us, and frustrates us, and angers us to the point of wanting to be unkind in return--that very weakness may be a wonderful strength under normal circumstances!" And if we could know the other side of that person who is so frustrating, perhaps from knowing their strength, we could allow for their weakness.