Few people are unscathed when a major event happens in their life. They suddenly begin to look at what really makes them happy. Some people find they would rather be single and stay that way. The realization hits them that they have never really been happy in a relationship. Rather than feeling supported, trapped is a better word to describe their feelings. The major life event has given them the courage to find their true needs in life.
Getting out a relationship is not always easy. They have to face the fact they are creating sadness and hurt in their partner's life. This is especially true when the relationship has been a long term one. While it may hurt their partner badly in the short term, both will be better off in the long run. Each will be able to live their lives better. The partner being left will be able to find someone that really can be happy in a relationship.
Being alone is not the same as being lonely. Loners tend to enjoy and cherish their time without other people. One of the drawbacks of this is losing the physically intimate part of a relationship. Fuck buddies are a perfect substitute for this need. A person that functions well as a fuck buddy may also be a loner that does not want to be trapped in a relationship. They acknowledge their physical needs as well as their craving for freedom.
There are a great many people that find themselves in this situation. For some, it may be a permanent condition. Others may just need space for a short time and then get back into dating and a relationship. In the meantime, no strings attached sex takes care of physical needs without the entanglement of a commitment.