Here's why:
* Pigs love mud: Pigs are known for their love of wallowing in mud. The mud cools them down, protects their skin from sun and insects, and helps them stay clean.
* Contentment: When a pig is wallowing in mud, it's clearly enjoying itself and feeling relaxed and comfortable. This image is associated with pure happiness and contentment.
So, when someone says they're "happy as a pig in mud", they are expressing a feeling of deep satisfaction and joy.