In Book 15, Telemachus is still visiting Menelaus in Sparta and has no idea that Odysseus has already returned to Ithaka in disguise, and Athena must take steps to ensure that Telemachus is re-united with Odysseus and Penelope as soon as possible. She tells Telemachus that he must leave for Ithaca immediately:
. . . it is wrong for you to remain here a moment longer, leaving your property unprotected, with such lawless men [suitors] in your house . . . . (15:8–11)
At this point in the story, Telemachus has heard enough from both Menelaus and Nestor to make him believe that his father, Odysseus, may still be alive, but Athena wisely uses his self-interest to encourage him to return—that is, to protect his property. Further endangering his property is Penelope, who, according to Athena, is being encouraged to marry one of the suitors:
. . . and might carry off some of your treasures without your permission. You know how a woman is: she wants to enrich the man who is marrying her, and she never bothers to think about her first husband once he is dead . . . . (15:17–22)
Athena cleverly plays on Telemachus's filial emotions, as well as the strained relationship he has had with Penelope, to get him moving quickly to return to Ithaca in order to protect his personal property (as Odysseus's heir) and, perhaps more important, to keep Penelope from marrying a suitor when there is a possibility that Odysseus is alive.
A second reason, also based on self interest, is that Athena tells Telemachus that the principal suitors are planning to ambush him on his way back to Ithaca:
Some of the suitors—the ringleaders—are lying in ambush for you between Same and Ithaca . . . and they mean to kill you on your way home. (15:27–29)
This warning from Athena gives Telemachus a good reason for leaving the hospitality of Menelaus's court, and he uses the danger of losing his property and of being killed as his justification for his abrupt departure from Menelaus—otherwise, his departure would seem to be a violation of Menelaus's xenia, his overwhelming hospitality, a sacred obligation of a host in Bronze Age Greek society.
The decision to return to Ithaka, like many decisions made by humans in the Odyssey, is brought about through the interference of the gods, a characteristic of the Homeric epic that we see repeatedly in both the Odyssey and in the Iliad—much of the action occurs not from independence of mankind but through the actions of various gods. In this case, fortunately, Athena has the best interests of Odysseus, Penelope, and Telemachus in mind.
While Telemachus is in Sparta at the palace of Menelaus, the goddess Athena appears to him in a dream. Athena informs him that Penelope might remarry, jeopardizing Telemachus's inheritance. Athena urges Telemachus to return to Ithaca, and he complies.
Telemachus is responding to Athena's warning out of a sense of duty to his mother as well as his own desire to preserve his wealth. Athena is setting the stage for Telemachus to reunite with his father, Odysseus.