I can try to offer some potential reasons that might explain your friend's behavior. It's essential to remember that these are only speculative possibilities, and the actual reason can vary based on the individuals involved.
Lack of empathy or consideration: Your friend might not have fully considered how her actions would affect you emotionally. She may have been focused on her own desires and needs at the time and didn't take your feelings into account.
Conflicting emotions: People can be complex, and sometimes they may struggle with their feelings. Your friend might have been attracted to your ex and acted on those feelings, even though she was trying to support you in winning him back. Emotions can be confusing and sometimes lead to conflicting behaviors.
Fear of confrontation: Your friend might have been afraid of how you would react if she told you about her intentions or actions. She might have chosen to keep it a secret to avoid potential conflicts or losing your friendship.
Low self-esteem or insecurity: In some cases, people might act in ways they know are hurtful to others because they struggle with their own self-esteem or sense of self-worth. By hooking up with your ex, she might have been seeking validation or trying to feel wanted.
Lack of boundaries: Some people may not have a clear understanding of appropriate boundaries in relationships. Your friend might not have realized that pursuing your ex behind your back was crossing a line and hurting you in the process.
Momentary lapse in judgment: Sometimes, people make impulsive decisions without fully thinking about the consequences. Your friend might have acted on a whim without considering the potential fallout.
It's essential to communicate openly with your friend about how her actions have affected you. Try to approach the conversation calmly and express your feelings without accusations. It's possible that she might not have realized the impact of her actions or might be remorseful for what happened.
However, if this behavior is part of a pattern of hurtful actions, you might want to reevaluate the trust and compatibility in your friendship. Trust is a crucial element in any relationship, and it's essential to surround yourself with people who respect your feelings and treat you with care.