There are very few people that have never experienced jealousy when dating or in a relationship. It often has nothing to do with their partner. It is an expression of their feelings of inadequacy at some point. It may be that they are having a difficult time at work. They will suddenly feel they cannot handle work, and this leads them to believe they are not good enough for their significant other. This may give them the impression that the person is looking around for a new partner.
Feelings like this are generally transitory if the relationship is solid. It is important to discuss these feelings with a partner if the issue arises on a regular basis due to issues outside the relationship. A partner may have the same feelings when their life is not going well. If a couple can work through these types of feelings, it will strengthen their bond. For those that either refuse to acknowledge these feelings or have them often, there may be other problems within the relationship.