I didn't understand why Erta, or concentrated goodness, would make Toby and Marie both think of happier times, but am happy to leave that as a 'why not?' of the story.
The way I understood it, on a very abstract level, is that Erta tastes like happiness. Sensory associations are one of the biggest keys to memory. I've heard smell is the strongest, but that's closely linked to taste, so not a big stretch. Natural flavors (i.e. flavors that actually exist outside of this story) can have strong associations, Alasdair listed a few of these. None of these have any kind of universal reaction across humanity--they may be associated with nothing at all, or with some powerful childhood memory for good or bad. Erta, on the other hand, has an unnatural origin and works in a different way. It works through the sense of taste, but taste is nothing but a vector for the emotional association. So instead of tasting cherries and reliving a memory associated with cherry flavor, the Erta tastes of happiness and makes you relive a memory associated with happiness.
You've heard the expression "comfort food"? Which is often things that are typically served for holidays, like roast turkey, mashed potatoes, cranberry sauce, etc... Though they taste super delicious, they also have strong emotional associations across large swaths of society with happy times spent with family. That's about as close to this sort of universal emotional association food as I've ever heard of, and it's at best incomplete--It's assuming 2 things that are not universally true:
1. That your family cooked those particular things on holidays.
2. Your holidays with your family were a happy memory.
Erta bypasses these assumptions entirely, assuming no particular personal history, but jumping right to the "happiness" reaction. No wonder this restaurant is wildly popular.