There could be several reasons why your ex repeatedly praises his new girlfriend when speaking to you, even if you don't respond:
Seeking validation or attention: He might be trying to elicit a reaction from you by talking about his new girlfriend in a positive light. Praising her could be his way of seeking validation or trying to make you jealous.
Insecurity: Your ex might feel insecure about the breakup and want to showcase his new relationship as a way to prove that he has moved on and is doing well without you.
Emotional immaturity: If the breakup was difficult for him, he might be using these conversations to cope with his emotions or to show that he has "moved on" to a seemingly better situation.
Comparison: By praising his new girlfriend, he might be indirectly comparing her to you, either as a way to make himself feel better or to indirectly communicate that he's found someone better.
Defensiveness: If he gets mad that you don't react, it could be a sign of defensiveness. He might be trying to overcompensate and prove to himself (and possibly to you) that he's happy with his new partner.
Trying to make you jealous: Some individuals try to incite jealousy in their ex-partners to gain a reaction or to make them reconsider the breakup.
Maintaining a connection: By sharing details about his life, including his new relationship, he might be attempting to maintain some level of connection with you, even if it's just through one-sided communication.
Regardless of the reason, it's essential to remember that you have the right to set boundaries and not engage in conversations that make you uncomfortable. If his actions are bothering you or affecting your well-being, consider limiting or cutting off communication with him. Focus on your own healing and growth during this time, and prioritize your emotional well-being over any pressure to react or respond to his behavior.