So, i think the sadness of the pure intellect comes from the fact that being "stuck" in the intellect (thoughts) level is a barrier to our evolution.
Think about people who are only moved by emotions, without rational thinking to moderate it. There are scores of persons like this. And their lifes may seem interesting to us, but they are leaves in the wind. Without the intellect to moderate their emotions, they just go from heaven to hell almost instantly, under the influence of their emotions. And i would never think that being like this is better than being a rational person.
The anachronistic thing about stupidity is it's tendency to induce unnecessary complications (of mind and circumstance). This evidence that rational thought is "better" glares obvious all the time, like when you need to do your taxes or work through relationship issues.

However, I still feel left in the dark as to the nature of the smart/stupid. The dictionary defines "intelligence" as "capacity for learning, reasoning, understanding, and similar forms of mental activity; aptitude in grasping truths, relationships, facts, meanings, etc." and gives no insight as to the reason for the difference in this capacity among individuals. In contrast to bhouncy's belief, I feel that this "capacity" is integral to the level of happiness and the very meaning of what happiness is to a person.

Well... in my humble opinion, its an unwise belief. We humans have (at least) four ways to percieve and interact with the world. From the most primitive to the most evolved, they are:

Physical sensation
Emotions
Thoughts
Spiritual insight

When we are born, all we know are the physical sensations. The comfort of our moms lap, the pleasure of eating, etc... when we grow up a bit, we start to percieve and feel emotions. Little children has so much emotion... years later, we start to think thoughts. And only after decades, only some few people attains spiritual insight.
I don't get how you can back up what you're saying here. How can you know that newborn babies don't experience emotions or thoughts? The pleasure of eating is both a physical sensation and an emotion that you attach to that sensation. It also would logically require the phenomenon of thought to be cognizant of the food and your mouth and taste buds and your own existence (you have to know you exist to know you want to eat food in the first place). The very act of recognizing one's own role as a perceiving ego is spiritual insight, and it is the most basic foundation of all the other modes of perception. Perhaps not everyone realizes the strangeness and significance of the fact that inside this weird lumpy tall hairy organic system there is a seemingly magical thing that perceives (and orders and thinks about and essentially creates) all these things that surround and affect the wet, soft, strangely shaped things that are our physical bodies.