User blog comment:TheCarrotSaysYumYum/Dragons, People and Demons.../@comment-115.249.242.137-20120721144310

Igneel and other Dragons must have fought battle with Zeref before their disappearance, and Igneel must have said to Zeref, "You will have u r defeat by Natsu."

Therefore, Zeref has high hopes to be defeated by Natsu.

Come to think of Gajeel, he was made to join into the guild only beacause Makrov felt sorry for him.