It stops you from thinking. By doing this it improves mood, as your thoughts do not interfere with your natural emotions and drag them down. You also tend to have no will when you are on diazepam, and if you take a very small amount of it, you find that you are able to have much greater muscle control as you attain stillness without interference from the tiny tremors that are caused by nervous tension. Great if you are a rifle shot, superb if you are preparing for a martial arts kata, really really bad if you are a surgeon or you are driving.
There seems to be an inverse correlation between intelligence and happiness implied here. But I doubt that is the fault of the chemical. Like all drugs its use must be carefully controlled because of medical side effects, in this case addiction being the primary risk.