Jealousy can be a good thing, and it’s really important for a relationship to function normally.
What you must learn is to distinguish between the real definition, unhealthy jealousy and healthy jealousy.
Do I confuse you?                                                                                                                     I understand if I do. But you’ll know what I mean in a minute. We primarily tend to focus on the unhealthy instead of the healthy.

An example of unhealthy jealousy may be that you are checking your partner’s phone when he/she does not see it.
Or that you get angry and jealous, when your partner is going to a party, because you are convinced that he/she maybe will do something inappropriate behind your back.
Both examples are enormously destructive for you, your partner and your relationship.
You might get angry and upset, and your partner will be the same at you, because your lack of trust and your partner will not accept that you are getting jealous and angry over nothing. Here’s the difference then:
If your partner actual cross your personal boundaries. For example, this could be if he/she is texting with an ex boy- or girlfriend, which you both have agreed is crossing your personal borders, so it’s important if you make a stand, and it’s easier to manage if you do get jealous.
Therefore it is important that you both are familiar with each other’s personal limits to define what is okay and what’s not for both of you.

Another example of healthy jealousy could be if your partner has danced a little too cheeky with someone at a nightclub and you find out. Then it’s okay to be jealous, because you have a reason for being so. That’s called healthy jealousy.
Now I hope you got the point.

It’s important to be jealous, when you really have a reason for being so, it’s a completely trivial sense we humans get, if we feel unfairly treated. We do this to take care of ourselves, and to leave the partners which aren’t good for us.
Think about, if you never became jealous, and therefore never did anything about it. Then you wouldn’t be able to say no, and then the risk of being treated unfairly by your partner would be much bigger.

Besides, it’s naturally impossible never to become jealous just once in your life, jealousy is the feeling that lies close to love.
If you weren’t ever jealous, you couldn’t set your own limits and your partners, and that’s necessary in order to have a healthy relationship.
If your partner did something that crosses your limits, it’s important that you can say no and show what your worth, because you won’t accept to be treated like that.

Therefore unhealthy jealousy is actually better than healthy jealousy, because you can change the unhealthy jealousy and your partner can’t. It’s conversely with healthy jealousy.
You can also categorize the two types of jealousy as true and false.

Remember that jealousy is an emotion, and as with all other less fun emotions, they come when your body reacts to something, and the red alarm inside you is reacting.
So your jealousy is just protecting you as long you can control it.