And thinking about food makes people want food more, it doesn't satisfy them. Somehow attaching "wants" not "likes".

If I had to make a couple guesses at the differences, the first would be that Yvain knows that learning Swahili is way more work than its worth for that amount of pleasure and that the reality check keeps him from getting too motivated to do it. Another factor could be that he doesn't go learn Swahili right then like you would for eating or cleaning your desk.

A more speculative guess would be that he's imagining it without somehow inhibiting the signal that the goal is satisfied. Some weak predictions might be things like "I have to remind myself that I didn't actually learn swahili". It seems like this is the angle he's going for, without mention of the possibility of inhibiting that response in other cases.