One of the more important parts of being in a relationship is the ability to trust a partner, and insecurity can cause a person to become jealous. They see someone speaking to the one they love, and they have a sudden feeling there is something going on between the two. Accusations begin to fly, and the pair end up hurting each other with words hurled in anger. This is actually a fairly common scenario that occurs when one person does not have the confidence to feel worthy of their partner.
People who cheat tend to be smart enough to hide their activity from their partner, and they enjoy sneaking around. Their goal might be to get caught, but many of them feel superior when they get away with it. These are not the people who worry about a partner with jealousy issues, but they tend to be the ones who have them. They know what they are doing is wrong, and a partner who speaks with someone else might be doing it too.
Self-esteem has long been a relationship issue, and partners who feel unworthy are always wondering when their significant other will wise up and leave them. It causes them to look at the dark side of any encounter with others, and their suspicions grow as the relationship lingers. They are always questioning their partner about activities when they were apart, and pointing out even small inconsistencies is part of how their jealousy manifests.
The lack of trust between two people does not make for a good or lasting relationship, so it is often a deal breaker for two people who want to build a life together. While counseling can help, few people are willing to work through the long process. Splitting up generally occurs within a few months to a year, and the jealousy person has only their own actions to blame.