So Jellybean plays practical jokes on all the other beans. (Spoilers ahead) They don't like "bean" messed with so they all play a joke on Jellybean and pretend to forget his birthday. Jellybean learns his lesson which is that it sucks "bean" on the receiving end of a joke. So if he actually does take this lesson to heart, his entire identity and reason for living is crushed.
On a side note, Vanilla Bean bakes him a b-day cake. I was a little disturbed thinking she either used part of her body to flavor it or got "vanilla extract" from a relative she murdered, but when they show the cake cut, it's obviously chocolate. But that makes me realize that I've never read about a Cocoa Bean in any of the books...