"But go and learn what this means: 'I desire mercy, not sacrifice.' For I have not come to call the righteous, but sinners." ~ Jesus, Matthew 9:13
"But the tax collector stood at a distance. He would not even look up to heaven, but beat his breast and said, 'God, have mercy on me, a sinner.' " ~ Jesus, Luke 18:13
"But for that very reason I was shown mercy so that in me, the worst of sinners, Christ Jesus might display his unlimited patience as an example for those who would believe on him and receive eternal life." ~ Paul, 1 Timothy 1:16
Did you catch the keywords?
I think they begin with "but" because they state a truth that is in contrast to the world's view and behavior. That is, to my view and behavior, all too often.
When I think I've sacrificed, when I'm tempted to boast to God in prayer about all I've done for Him, when I'm convinced that I've been doing the right thing in the right way and for the right reason ... I need to be reminded Whose perfection and sacrifice did everthing for me that I could not do.
I need to be reminded that - though forgiven - I am still a sinner, and that I should show what I have been shown; give what I have been given: mercy, faith, patience, an example.