Fun fact: cats don’t just purr when they’re happy! They use it as a method of self-calming and healing when feeling anxious or unwell. Amazingly they purr at a frequency that stimulates healing, which is one possible reason as to why the behavior evolved in the first place. Each individual is different of course, but generally you can get a better sense of a cat’s emotional state by observing their eyes (that’s how cats smile!) than by whether they are purring or not.

https://www.petmd.com/cat/behavior/why-do-cats-purr

https://www.usatoday.com/story/news/2023/05/05/do-cats-smile/11761709002/