Because God says so:

 

Write this down, for these words are trustworthy and true:

To the thirsty I will give water without cost from the spring of the water of life - freely, as a gift.

                                                                                                (Revelation 21:5-6)

 

 

Write the vision
And make it plain on tablets,
That he may run who reads it."
"

(Habakkuk 2:2-3, NKJV)

 

 

As you hope in Me I will renew your strength. You will soar on wings like eagles; you will run and not grow weary, you will walk and not be faint.                                   (Isaiah 40:31)