"Sincerity can't be exchanged for sincerity, so let's be happy together" means that he has a heart-to-heart relationship with you, so since he can't exchange your sincerity, he doesn't care and won't be honest with you again.

I once met such a friend:

We are very happy together, which can also be understood as having common hobbies and ambitions. But unfortunately, he took all kinds of precautions against me.

I am good at science. He often asks me some scientific problems, and I am willing to discuss them with him, telling him sincerely how to do it, how to further analyze such problems and so on. Moreover, I am not good at liberal arts, such as reading and analysis, so I go to him occasionally if I don't understand. At first, he always faltered, as if listening to me carefully, but in fact he was absent-minded and didn't say anything at last.

I didn't care once or twice, but after several times, I knew that he didn't really want to help me, maybe he was afraid that my grades would surpass him.

Therefore, to make friends, you must find the right person, and you can only communicate if you really change your mind. But if everyone is playing with their brains, there is no need to continue.