Nietzsche ridiculed the Socratic equation “reason = virtue = happiness,” calling it “weird” and decadent, but in fact one can consider Nietzsche’s ridicule itself to be weird and decadent. It is of a piece with his inability to see the value of the altruistic Christian morality, the psychological profundity of which should be obvious. Happiness is based on high self-esteem, which requires that you see your implicit self-love confirmed in the world, reflected in people’s behavior towards you, validated by yourself and others. Ordinarily, acting virtuously will get you not only self-respect but also the respect and affection of others—which will itself increase your self-respect, and so your happiness. In those cases in which doing the right thing gets you ostracized or punished, you will still at least have the happiness of knowing your own value, and your superiority to the contemptible herd. Conversely, it is well known that dissolute living or obsessive chasing after fame and money does not equal stable happiness, because it does not entail a secure sense of self. So, inasmuch as it is reasonable to do what makes you happy, it is reasonable to do what is right and virtuous.
Moreover, the pleasures of reason—science, philosophy, inquiry and creativity—can be definitely happiness-inducing, even if at times an obsessive engagement with philosophy or science can be tormenting. Doubtless there is some truth to the cliché that ignorance is bliss; for example, ignorantly believing in God—the idea of which reason rejects—can make people happy, and understanding the “meaninglessness” of human existence isn’t necessarily a happy experience. It can drive to despair. In fact, however, it is never just a thought (like the thought of God’s nonexistence) that makes someone unhappy; it is a thought conceived in certain circumstances, for example conditions of social alienation or loneliness or lack of interest in life. It isn’t reason or knowledge that makes unhappy, as so many poets and others have thought; it is a particular set of experiences, which cause you to latch onto some given thought and obsess over it, imagining that it is the source of your unhappiness. In a healthy social environment, it’s perfectly possible not to be disturbed by some truth that a person in a different social context might find disturbing. –In any case, it’s a risky strategy to pin your happiness on a lie, like the lie that God exists, because doubt is apt to seep in and upset your carefully arranged house of illusions.
In short, the Socratic equation is not only morally uplifting but psychologically valid, all things considered. One can object to it on the grounds that it optimistically oversimplifies, given that virtuous people are not necessarily happy—and vice versa—and “rational” people are not necessarily virtuous or happy, etc. But on the level of ideal-types—and/or in a sanely organized society—there does seem to be an elective affinity between the three concepts of reason, morality, and happiness. Enlightenment thinkers thought so. And the Enlightenment is not easily called a “decadent” movement; it was too naïve, confident, optimistic, and vigorous for that. What is decadent, perhaps, is a late-nineteenth-century rejection of something as universal and “clean” as the Socratic and Enlightenment association of reason, morality, and happiness—for such a (Nietzschean) rejection proceeds from a faux sophistication, a disgust with optimism, a fondness for perverse formulations, a late-romantic or fin-de-siècle fascination with the irrational and the repressed. –One should strive to be rational and moral, and relative happiness may well ensue.
 One can also object that, contrary to Kant’s philosophy, morality cannot be grounded in pure reason alone, and so in this sense morality ≠ reason. I think that this Humean and Nietzschean objection is correct, but to defend it would require a technical discussion inappropriate here.