I asked my girlfriend, who is Dutch, if she likes it when people say "eet smakelijk." She said "no, but you have to."

"Eet smakelijk" is the Dutch equivalent of "bon appétit." It seems as if every European culture has a variant. Spain, yes; Germany, yes. I've asked people from Italy, Poland, and Sweden — they all have a form of it.

The usage is quite specific, though of course there are regional variations. (In France, somebody wished K_ "bon appétit" when bringing her a cup of coffee.) But, whatever the differences, the phrase is part of a strong social code. Within the specific local custom of use, the application of this expression is compulsory. Generally, if you see somebody eating, and they are within that realm of familiarity particular to the local custom, you must wish them a good meal.

You'd say "enjoy your meal" in literal English-language translation. But literal translation doesn't really work, because there's no corresponding tradition. (A Spanish friend suggested that this is because the English have no cuisine. Speculation, but provocative....)

It doesn't necessarily sound right, in my language, to wish somebody a good meal just because you see them with food. In any case, it is not necessary in English-speaking cultures — not compulsory, and not normal.

But in nearly every other European language (if not every one,) it's important to wish an enjoyable meal. It's often necessary to do so, in order to be polite, even if you were unlikely to speak otherwise.

You just have to accept it. You don't have to like it. You don't have to do it; but it may be awkward if you don't.