Disregarding the outside influence of the author and concentrating solely on the poem and its characters as a fictional reality, there are several ways of interpreting this choice. Anchises is the one who leads Aeneas and the Sibyl to the false gate. Does he know that his son does not truly have these things as his goals? Or does Aeneas simply choose the false gate because he doesn't know any better? To further our theory that Aeneas is a bad man, we shall go with the interpretation that Aeneas left Tartarus by the false gate because he did not truly value the goals that had been set forth for him by the gods as his real goals and dreams.
 | When Aeneas first enters the underworld, he is ignorant of the specifics of his destiny. He knows that he must found a city in Italy, but not what will become of that city. Aeneas meets three significant people in the underworld, each of which represent some important aspect of himself. The first is Dido. She represents his role as a lover, a role which he seemed to leave behind in Troy with the death of Creusa. But he found love against the odds. However, his destiny can not allow him to feel love, for that love would prevent him from fufilling his great task. The first aspect of himself that Aeneas must then give up is his role as a lover and husband. He leaves this behind when Dido shuns him, as well she should. |
A side note here about Aeneas as a bad man. When he sees the shade of Dido in the underworld, instead of profusely apologizing, he spouts more of his "Fate commanded me" rhetoric. Even if this is true, now is not the time for justification of his actions. Now is the time for grovelling. Even in the Underworld to a hapless shade, he can not say that he is sorry.
| The second figure that Aeneas meets is that of Deiphobus. Deiphobus was a fellow Trojan and soldier who was slain horribly at the last battle. He and Aeneas talk for a few hours about what happened to him at Troy, and then Deiphobus pardons Aeneas for not burying his actual corpse and simply wailing over an empty tomb on a faraway island. Deiphobus represents Aeneas' past life as a Trojan and survivor of war. So the next aspect he leaves behind is that of his Trojanhood, but more importantly the aspect of him which is a conquered person. No more can he wander the Mediterranean seeking help. |  |
From now on, that tacit will not work for him. For example, when he sends an embassy to beg Latinus for help, war breaks out as a result. And when he seeks help from Evander, Pallas, Evander's son, is indirectly killed as a result. Although Aeneas does not realise it yet, he has lost a small piece of himself here, a small part of his identity has remained with Deiphobus. The third and final person Aeneas meets in the Underworld is his father, Anchises. Here, Aeneas learns the specifics of his fates when Anchises points out the souls of the future Romans who are waiting to be born in the Elysian Fields.
 | Anchises is very keen on Aeneas as the founder of the Roman race. He is most thankful that Aeneas was not sidelined permanently by Dido(grrrrrrr) Anyway, Anchises represents the aspect of Aeneas that is a son. He must leave behind once and for all these familial ties, seek his father's advice for the last time. (You might also notice that Iulus becomes less prominent of a character after this part, and is never mentioned as being with Aeneas in the same way as when he clung to him in Libya again.) Aeneas keeps losing parts of himself, and with his father so eager to have him take on his destined role, he easily loses his son aspect. |
So when Aeneas goes up through the gate of false dreams he does so because he is no longer Aeneas. The real Aeneas, let's face it, would have preferred to stay in Troy. Indeed he says as much to Dido. Barring that, I believe he truly wanted to stay with Dido in Libya. Unfortunately he was not brave enough to even attempt to defy the gods. Perhaps it is only the passage of time which makes us look on Aeneas as a bad man. We do not comprehend such piety. But then we have the examples of Odysseus, who defied Poseidon and won. In comparision, Aeneas seems an empty shell of a person.
"The real subject of the Aeneid is not Aeneas... it is Rome and the glories of her empire, seen as the romanticist sees the great past. The first title given it was The Deeds of the Roman People. Aeneas is important because he carries Rome's destiny; he is to be her founder by the high decrees of fate."(Edith Hamilton, The Roman Way, 1960)
Aeneas easily gives up his aspects in the Underworld, transforming completely into the 'Roman Aeneas' from the 'Trojan Aeneas' No more does he strive for what he as a person wants, but for what he as a cipher for the gods should want. Thus he goes through the gate of false dreams, for these dreams are not his true dreams, but the ones he has been told he should have. And this is the crux of why Aeneas is such a moron. He does not adapt himself to his destiny, but instead loses himself to it.