Why some people put ash on their forehead?

In many Indian traditions we put mud on our foreheads & here is why...
1. To pay respect to the land we live on
2. For gratitude to Earth for giving us food
3. Its the grossest of the 5 elements that we can see, feel, smell & touch. 

Apart from patriotism & gratitude , its a reminder that we will merge back into Earth.

Also Catholics have Ash Wednesday as a tradition to mark beginning of Lent.

Ashes are a sign of physical death, as in 'ashes to ashes, dust to dust.' We began as dust (a joyless and lifeless existence), and our bodies will return to dust until we are raised up by God. By receiving ashes and keeping them on, we publicly proclaim our intent to die to our worldly desires and live even more in God's image.

Many Hindu's apply ash every morning as a daily reminder of the above.