Even putting aside atrocities, sometimes we deprive ourselves of perfectly good pleasures, including those involving love and companionship, because of the decisions of the long-term self. Think of the workaholic who never sees his children, or the anorexic who denies herself the pleasure of food.
What motivates the short-term self? Some of its pleasures are easy enough to explain: sex and food and friendship are evolutionary no-brainers, we enjoy them because they motivate us in adaptively useful ways. But others are more mysterious.