If you’re sensitive to caffeine, it can cause irritability or anxiety in high doses (and what’s "high" varies from person to person). How? Chemically, caffeine looks a lot like adenosine, a "slow-down" brain chemical associated with sleep and relaxation of blood vessels. Caffeine binds to adenosine receptors on nerve cells, leaving no room for adenosine to get in—so nerve cell activity speeds up, blood vessels constrict—and you get a caffeine buzz (or irritable jitters).
Of course, if you caffeinate yourself daily, you’ll likely develop tolerance to its effects and the jitters will subside. But that also means that eventually you’ll need a regular caffeine fix just to reach your baseline level of alertness. And your body will adapt by producing more adenosine receptors, making you more sensitive to the effects of adenosine. So if you don’t have your daily cup, you’ll likely develop withdrawal symptoms like extreme fatigue and splitting headaches (caused by constricted blood vessels).