Insecurities and the lack of thereof when it comes to comparing someone else life to their own

Insecurities

People do

Insecurity in my opinion!

The success of someone else

Number of things

Insecurity.

That's a
feeling we need to pay attention to..
This can cause many bad decisions or something you can't take back.

It's object we lack of or don't have that you see other that have.
It can be emotion problem too.

People don’t normally experience jealousy unless they feel threatened by another person or entity. Sibling jealousy is usually caused by a child’s fear that the parents will replace him or her with a new sibling or love another sibling more. In romantic relationships, jealousy is typically triggered by a third party. The third party doesn’t have to actually pose a threat; the mere perception of a threat is enough to get the wheels of jealousy turning.

When you're too buddy buddy with someone who has a partner