Often, I find myself leaning over a table, telling people to enjoy the food however they're comfortable, but if they want to eat the Thai way...
And I start telling them which utensil goes in which hand, and a couple basics of Thai etiquette, some of which I learned from a quaint midcentury book on Thai etiquette for the upper classes, and some of which I learned while living in Bangkok or studying in villages, making mistakes and being corrected. I'm still not sure if I used the right spoon(s) that first night in Mae Chaem, or if I both stuck my own spoon into everything and ate from the serving spoons. Nothing that really effects the enjoyment of the food itself, but that ostensibly helps get it from the serving plate to one's mouth in an agreeable way.
Thai meals, when they aren't quick lunches of noodles or single-plate dishes, are centered around rice and shared dishes, family style. One's plate of rice is one's own, and everything else is shared: curries, soups, salads, etc.
Some dishes are completely absent from the menus of Thai restaurants in America, and even Thai restaurants in Thailand that serve foreigners. The most notable is the entire category of relishes: naam phrik, lon, and related sauces, dips, and condiments. These dishes, for which 'relish' seems the most reasonable English word, (and is used by writers on Thai food such as David Thompson and Leela Punyaratabandhu) are absent for a few reasons. One, I'd surmise, is that restaurateurs fear they are too pungent, too spicy, too exotic for non-Thai diners to enjoy. Certainly my experience is that relishes such as naam phrik thua phat, a fried relish of chile and peanuts, and lon taw jiaw, a simmered coconut cream relish with salted soybeans and shrimp, are not only approachable, people can't get enough of them.
A more practical reason is that non-Thai people don't know how to eat them; at least that's phii Leela's view. Relishes are served with a variety of garnishes, which are eaten with the relish and, above all, rice. But how? Do you dip the cucumber in the chile relish? Do you eat a spoonful of lon, curry-like, with rice?
Historically, Thai people ate with the right hand, like everyone in South and Southeast Asia did (and in many places still do). To this day, in more remote parts of the North and Northeast - sticky rice country - people eat everything from soups to stir-fries with their fingers, a bit of rice, and a remarkable amount of grace. I learned to at this way in a remote village, and since then I've been told that I eat like an old person, when a Thai date saw me sitting on the floor, molding sticky rice in my right hand to pick up some papaya salad: younger people simple tear off a chunk of rice to eat, and use utensils for the accompanying dishes.
During the 19th and 20th centuries, when European countries were colonizing Southeast Asia, a number of changes took place.
The court has always been among the first points of contact with the outside world, and forks and spoons may have been introduced at this time, possibly as a way to demonstrate to the encroaching French and British that Thailand was 'civilized' to the extent that colonization would be highly inappropriate. Or, it just became fashionable. We know that Western cooking had become faddish with the upper classes of Thai society, and adopting the foreign utensils may have simply been a related mark of sophistication.
Whatever the reason, Thai dining progressed from the hand to the Chinese soup spoon to the fork/spoon pair, which obtains today (Chinese foods, notably noodles, are eaten with chopsticks and a Chinese spoon). Etiquette demands serving utensils, but casually family or close friends might dip into the serving dishes with their own spoons. At the end of the meal, the spoon and fork are placed together on the plate, indicating that one has finished eating.