Hi Angela,
When two people get jealous after seeing the other talking or getting too friendly with a member of the opposite sex, it often means that they both feel possessive of each other.
In other words, both of them want to be at the receiving end of attention from the other with no one else getting in the way.
Even though they haven’t confessed any feelings for each other and they are not friends, there is a part of both of them that hopes that through some fortunate circumstance, they will be able to become acquainted.
However, neither one of them is willing to make that first move because they would much rather have the other risk the possibility of rejection.