The Happy Gland is an emotion that causes happiness.
It is stated to be inside Squidward's Brain. Every time Squidward sees SpongeBob and/or Patrick, dark storm clouds form inside Squidward's brain, causing the Happy Gland to get the flu, thus it is not able to keep Squidward happy. The only appearance of it is in "Breath of Fresh Squidward," but, due to Squidward's occasional happiness, (if real) it is used. When the storm comes, the Happy Gland tries to use his umbrella to shield itself by the storm, but the umbrella flies away which causes it to get the flu.