After having caffeine, I typically notice a change in my emotional state, becoming either moody and peevish or perky and happy. I’m fairly sure it has this effect on others, too. What I’m wondering is if caffeine acts as more of a “mood-intensifying” or a “mood-altering” substance, and if there have been any psychological studies on the effects of caffeine on emotion?
Can’t say as I’ve noticed such a thing myself and I’m a hardcore coffee addict. I typically drink three or four 20-24 oz. cups of the stuff a day.
I don’t know what the studies say but it certainly is a stimulant and as such can have an effect on mood. Alcohol can act as a mood intensifier as well.
Heavy (caffeine) drinkers probably have built up a maintenance level so that it has little effect on them–but watch what happens if they have to skip their coffee for a day!! 
I don’t think I’ve noticed the moody part (assuming I understand what you mean by that) due to caffeine, but I’ve definately noticed the elevated mood.