This is a very difficult question to answer...there are a number of reasons that spring to mind, including: being 'addicted' to sex and cheating because she feels she can't stop herself; the tragic impact of intoxication if she has a substance use problem (ie. she only does this when very drunk or under the influence of some other drug); she could be in a manic phase of bipolar mood disorder (this is an illness that needs to be diagnosed by a psychiatrist - there are other symptoms that would go with this, not just serial cheating); and she could also be retaliating against something in the relationship with her partner (possibly her partner cheating, abuse, demeaning behavior of other sorts). This is not an exhaustive list - but as you can see there is no simple answer without a thorough assessment.
Claire - SASHA
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