It displeased him, because of that affection which parents generally have for their first-born. See Genesis 21:11.
Related Commentaries of Genesis 48:17
Genesis 48:17
17 And when Joseph saw that his father laid his right hand upon the head of Ephraim, it displeasedc him: and he held up his father's hand, to remove it from Ephraim's head unto Manasseh's head.