If they are making each other jealous, they could be experiencing a connection that doesn’t require the both of them to be together, be friends or talk on a regular basis.
There are people who can develop an emotional connection with someone that they don’t know at all based on attraction and the stories that they have allowed themselves to make up in their minds about what that person must be like.
The stronger the attraction, the stronger the stories. This is often where the feeling of possessiveness comes in.
By this point, they have made up this other person’s persona, lifestyle, passions, dreams, history, etc.
In essence, they have created a surrogate version of this real life person in their minds. They become possessive of the persona that they have created.
Once they see this person talking or getting too friendly with a member of the opposite sex in real life, it goes against the fantasy that they have made themselves believe about who this person is and the connection that they both share.
This is often what will cause them to become jealous.