(With great zeal I have been zealous for the LORD God Almighty)
The prophet Elijah has was one of the most powerful prophets in the Old Testament. His miracles even rival those of Christ in terms of their grandeur and sheer awesomeness. Though of course no one can match Christ, Elijah comes very close. After having performed the miracle at Mt. Carmel in front of all the prophets of Baal and Ashera, proving to them once and for all the LORD is God, Elijah flees for his life. It's is strange to see a man so full of faith in God run for his life because of a woman named Jezebel (whose reputation precedes her).
But Elijah, more than anything, desires the LORD God almighty. He tells God that he is the only one left from amongst all of the Lord's prophets that have not turned or were killed by Jezebel. Elijah was alone, except for God. This is why Elijah so strongly desired the Lord, for he had no one else. Elijah also wanted to die as well, having had enough of life. (v4) It was in that moment God took care of Elijah, when everything seemed lost.
It is this phrase that embodied Elijah's life. When Elijah was all alone and clearly depressed, God gave him a new purpose and a disciple to take his place, Elisha. Depression didn't define Elijah, but it was his zeal for God.