There could be several reasons why a guy might want to see how you react when an ex expresses interest in getting back together with him. Here are some possibilities:
Testing your feelings: He might want to gauge your level of interest and commitment in the relationship. By observing your reaction to his ex's advances, he can get a sense of how much you care about him and the relationship.
Seeking validation: Some individuals seek validation from their current partner by showing that others find them desirable. If he sees that his ex is interested in him, he may feel validated and desired, which can boost his self-esteem.
Jealousy test: He might be trying to provoke a reaction from you to see if you become jealous or possessive. Some people use jealousy as a way to gauge their partner's level of attachment and interest.
Comparison with the ex: He could be comparing his past relationship with his ex to his current one with you. By observing your reaction, he may be trying to understand if his current relationship measures up to what he had before.
Uncertainty about his feelings: He might be unsure about his own feelings for you or still have unresolved emotions about his ex. By observing your reaction, he could be looking for clarity on his emotions.
Insecurity: If he is feeling insecure about the relationship or your commitment to each other, he may use this situation as a way to assess how much you value him.
Testing your trust: Some individuals test their partner's trust and loyalty by introducing situations that could potentially create tension or conflict. By seeing how you handle the situation, he might be assessing your trustworthiness.
It's essential to communicate openly with your partner if you feel uncomfortable or uncertain about their intentions. Healthy relationships are built on trust, communication, and mutual respect. If you find yourself in such a situation, have an honest conversation with your partner about your feelings and concerns. Understanding each other's perspectives can lead to a deeper connection and a stronger foundation for the relationship.