Why is Christ Distressed about Baptism? (Luke 12:49, 50)

We normally think that baptism is a wonderful sign and seal of the Lord's promise. It presents for us the certainty that Christ has endured an eternal punishment, and he has emerged triumphant. The very promise of Genesis 15 is confirmed in Christ's death and resurrection. Christ confirms the promise of the covenant of grace. We think of baptism being this sign of the covenantal promise.

However, Christ is actually anxious about his baptism and he desires to get through his baptism as soon as possible. Why is Christ so concerned about baptism when his father has already pronounced that he is pleased with his son? If Christ is already baptized then what baptism is Christ looking forward to undergoing?