In desert

Once, two friends went many days through desert.
And one day they argued, and one of them in a temper gave a slap in the face to another. His friend, felt a pain, but say nothing.
Silently, he wrote on sand: " Today my best friend has given me a slap in the face".
Friends continued to go, and in many days found an oasis in which they decided to bathe. That one who had got a slap in the face, nearly sank and his friend rescued him.
When he came to the senses, he cut on a stone: " Today my best friend has rescued my life ".
The first one asked him:
- When I offended you, you wrote on sand, and now you write on a stone. Why?
And the friend answered:
- When someone offends us, we should write it on sand that winds could erase it. But when someone does something good, we should cut it on a stone that any wind could not erase it.