It is easier to exclude harmful Passions than to rule over them, and to deny them admittance than to moderate them after they have been admitted. Seneca, On Anger
It is a mistake to think we are able to control our Passions after we have been foolish enough to indulge in them. Seneca writes, "As one who is cast headlong from a cliff has no control over his body... so it is with the mind if it plunges into Anger, Love, or the other Passions." The very weight of the Vice sends the mind tumbling downward, mixed up with Vice as though inside an ever-growing snowball rolling down a hill, increasing in size and speed as it goes, and picking up more Passions along the way. The lover, once having indulged in Love is later afflicted by it, and, as his immune system has been weakened, he becomes susceptible to all the other ailments associated with that disease, Lust, Jealously, Idleness, Intemperance, and Neglectfulness of duty. The angry man, besides suffering like an insane man under that most hideous of Vices, invites also Cruelty, Malice and Injustice. Indulge Fear and you run the risk of becoming a coward, and the coward is incapable of any honourable action.
One might object, "But what of Reason? Does not the Stoic school teach that Reason is mightier than the Passions? Shouldn't Reason, then, be able to control them?" Reason is indeed more powerful than Vice, and can always conquer the Passions - but not while she consorts with them! To win a battle, an army fights against, not alongside, the enemy. And it certainly does not invite the foe to become part of its command structure. Yet this is precisely what one does when he attempts to use his reasoning faculty while simultaneously welcoming any of the Passions. Reason will not be able to conquer those Passions, but she will become their slave.
If we would be free, we must block the Passions at the front gates. They are not harmless visitors. No matter what friendly or pleasurable disguise they might wear, their intention is to enslave us.