What is there to be jealous about? Is jealousy ever a constructive emotion? What does it reflect or define, individually? Is it ever reasonable? Is it resistible, inherent, or something else?

Your spouse.

Yes.

A bit of it is good for a relationship.

Of course.

Take your pick.

God is jealous, and God is perfect. So what are you talking about?

Jealous about?  Fear of losing someone you hold close to your heart.

a lot especially when you dont seem to have anything

Fear of losing your partner? If it is channeled correctly, it could be useful. Like realizing that you are not paying enough attention to your significant other.
Depends of how powerful is the reaction it can reflect things, if it is not too strong it could reflect care towards the significant other, but if it is too intense it can reflect that you are batshit crazy, a red flag for your partner or viceversa. It is irrational most of the times due to what I told you before about the hormones in the brain. It can make feel like you can lose the source of your well-being hormones. In other words, make you react like an addict that can lose his next dose.
I would say it is part of human nature. Few people can say that they are not jealous at all.

Thanks, Oscar! I always like hearing from you.