Aeneas himself, in the by A. S. Kline, explains his departure as follows:
it is Italy that Apollo of Grynium,
Italy, that the Lycian oracles, order me to take:
that is my desire, that is my country.
Later Virgil adds that
dutiful Aeneas, though he desired to ease her [Dido's] sadness
by comforting her and to turn aside pain with words, still,
with much sighing, and a heart shaken by the strength of her love,
followed the divine command, and returned to the fleet.
Thus, "divine command" -- obedience to the gods -- is the reason that Aeneas himself offers.