Marriage or a long term relationship is very important to many people, and some of them are willing to be with the wrong partner rather than spend their life alone. They see it as a way to achieve normality within their community, but the price they pay may eventually become too high. Living with the wrong person can be emotionally, mentally and physically taxing. Over time, the person who has made a commitment to the wrong person might eventually realize their life has little or no meaning.
Leaving a relationship is often seen as a rejection of the person being left, and it can be difficult to hurt someone in that manner. Even if the person is not the right partner, it does not necessarily mean they are bad or unlovable. It might simply be that they are not compatible, or their lifestyle might not be completely acceptable. Asking them to change is not always a possibility, and even that might be too much to expect after the relationship has formed.
Rejecting the wrong partner is more about life goals and self-esteem than anything else, but it is also a hurtful part of a bad relationship. The couple might not have any pressing issues or arguments, so rejecting the other person can be a complete surprise. They believe they have been a good partner, and they do not understand why they are being left. There are often no good reasons, so explaining the rejection becomes a condemnation of them.
There are no perfect partners, but there are partners who are just wrong. For those who have suddenly decided they want more out of a relationship, there is always counseling. If that does not work to bring the two together, then leaving is the only choice for at least one of them.