Jesus is demanding a radical kind of love, to forgive when they don’t deserve it. Peter’s question in v21 comes from the teaching in verses 15-20.  One of the new things I learned here was that the typical rabbinic consensus was that you would need to forgive a brother three times.  But Jesus radically challenges this idea.  Not three but seven times seventy! "Jesus does more than forbid vengeance; he transposes a cry of revenge (see Gen. 4:15, 23–24) into a call for forgiveness. Whether he says “seventy-seven times” (NIV) or “seventy times seven” (NIV marg.), his words reject a calculating, quantitative approach to forgiveness. Love within the church “is expressed by an indefatigable capacity to forgive the brethren.”25 The ensuing parable illustrates the lesson." (Chamblin, J. Knox. “Matthew.” Evangelical Commentary on the Bible.)