Caffeine is also a plant alkaloid, found in numerous plant species, where it acts as a natural pesticide that paralyzes and kills certain insects feeding upon them. Take a look at what it does to spiders, which is reflected in their web construction.
Caffeine is a drug that in large amounts, especially over an extended period of time, can lead to a condition termed "caffeinism." Caffeinism usually combines physical addiction with a wide range of unpleasant physical and mental conditions including nervousness, irritability, anxiety, tremulousness, muscle twitching (hyperreflexia), insomnia, and heart palpitations. (Under a rigid definition of addiction, meaning a process of escalating use, "caffeine dependency" would be a more descriptive term. However, under the widely accepted definition "chronic pattern of behavior that is perceived to be difficult to quit," caffeine may be said to be addictive.) Furthermore, because caffeine increases the production of stomach acid, high usage over time can lead to peptic ulcers, erosive esophagitis, and gastroesophageal reflux disease.
If these didn't convince you, then check out Top Reasons to be Caffeine-Free.
If they did, then check out How to Give up Coffee.