What is a "Personality Value"?

[Cassiel]

What is a Personality Value?
Why Wurmple needs a Personality Value before evolving into Silcoon/Cascoon?
Mine always evolve into Silcoon, but sometimes evolve into Cascoon..

Is it because their nature? Their gender? Or how to raise them?

Please answer thiz..

 

[blueparukia]

its completely random when you encounter it.
There is about a dozen threads on this in the R/S/E faqs section