Charles Ellicott Commentary 1 Samuel 18:16

"But all Israel and Judah loved David; for he went out and came in before them." — 1 Samuel 18:16 (ASV)

But all Israel and Judah. —This distinct mention of the two great later divisions of the chosen people seems to point to the fact that the compiler of the Books of Samuel lived after the final separation of the ten tribes from Judah and Benjamin, in the reign of Rehoboam. It is, however, clear from other notices (see, for instance, 1 Samuel 11:8; 1 Samuel 15:4, in this book) that at a period long before the final disruption between the north and south, a marked distinction between the two had begun to exist.

In David’s case, however, although he was of Judah, the future king was equally popular with the northern tribes.