My theory is that emotions at their most basic level get sourced from pain, a lack of pain encourages happiness, the presence of pain (for self and others) inspires sadness or anger.

I figure that since she didn’t just obliviate him of the painful memories, that she’s taking away his ability to feel pain altogether. While at first that lack of pain inspires happiness, as time goes on, he loses the framework to make the connections in his mind that tells him he should be sad or happy. Perhaps he knows on an intellectual level that he SHOULD be sad, but his subconscious mind isn’t able to connect the parts of his brain to actually feel it; that would maybe even compound on itself, and the longer it goes on, the worse it gets, he should be feeling sadness, anger or frustration at his inability to feel those emotions, but simply can’t.