Why does Lord Shamash urge Gilgamesh to attack Humbaba?

In the Epic of Gilgamesh, Lord Shamash urges Gilgamesh to attack Humbaba because Shamash strongly feels that Gilgamesh possesses the ability to defeat Humbaba in battle. Lord Shamash despises Humbaba and manipulates Gilgamesh into killing him. Gilgamesh wants to kill Humbaba in order to prove his bravery and fighting prowess. Lord Shamash, in turn, plays upon Gilgamesh's ego in order to force him into killing his arch enemy.