Greetings in Christ,
Why Ashes? It is a smudge of ash, hopefully in the sign of the cross, but not always. Imposed on our foreheads, it is a physical sign of the truth of our lives. We are dust and ashes. God created us out of the dust of the earth, and it is to that dust we all will one day return.
Since ancient times dust and ashes are a sign of repentance that is more than a passing, 'I'm sorry'. Dust and ashes are a sign that we will not deceive ourselves about our sin and its consequences. It is a confession that not only have we sinned but that sin, our sin, is a deadly thing.
St. Paul was not merely speaking in a metaphor when he wrote that the wages of sin is death. He was writing about the universal truth that sin always leads to death. This death is more than our bodies returning to the dust of the earth, it is separation from God for eternity.
Dust and ashes is our admission that we have sinned. It is our plea for the mercy that must come from God. In dust and ashes, we look to the cross whereon our Lord bore our sins to save and redeem us.