David said to him, “If you go on with me, you will be a burden to me.
Nothing, it seems, appeared to David more threatening in Absalom's plot than that Ahithophel was in it; for one good head, in such a design, is worth a thousand good hands. Absalom was himself no politician, but he had got one entirely in his interest that was, and would be the more dangerous because he had been all along acquainted with David's counsels and affairs...
Commenting on 2 Samuel 15:31-37
Unto whom David said,.... After he had heard what he had to say, and what tidings he brought: and if thou passest on with me; in his march and flight: then thou shalt be a burden to me; being to be maintained by him; and David having but scanty provisions, and so could not receive useless persons, as Hushai might be, perhaps an old man...
Then thou shalt be a burden unto me - It appears that Hushai was not a warrior, but was a wise, prudent, and discreet man, who could well serve David by gaining him intelligence of Absalom's conspiracy; and he directs him to form a strict confederacy with the priests Zadok and Abiathar, and to make use of their sons as couriers between Jerusalem and David's place of retreat.