Halpern makes the instructions for the dinner-for-one so specific because he wants to emphasize the fact that it is special. People generally tend to skimp on solitary meals on the principle that it isn't really isn't worth going to any great lengths to prepare and cook a meal just for one person. Halpern challenges this notion with his vision of substantial, indeed spectacular meal just for one. He is making the point that it is indeed worthwhile to lay on a special meal just for oneself. To this end, he draws up instructions not just for preparing the meal but also the surroundings:
This is the best part of the evening:
the food cooking, the armchair,
the book and bright flavor
of the chilled wine.
This picture of the comfortable, indeed luxurious environment is entirely in keeping with the picture of the richly satisfying (one might even say over-satisfying) meal. This splendid scene is entirely for the benefit of the solitary diner, not for family or friends or guests of any description. It is a glittering gathering of just one.
With this poem, Halpern draws attention to the issue of self-worth. He makes the point that, when alone, a person should not neglect him or herself; on the contrary, the single individual should lavish the kind of care, detail and thought on him/herself that he or she gives to others. Indeed, in the final line there is rather sardonic hint - in fact it is more than just a hint - that one's own company is preferable to that of others anyway. It is a somewhat ironic ending, perhaps, but it follows quite naturally from all that has gone before, the picture that builds up a sense of pampering and luxury for the solitary individual. There is a strong suggestion, indeed, that the door is deliberately barred against the rest of the world; others are not allowed admittance.
Think about how one typically eats by themselves. The goal is typically to finish the meal as quickly as possible usually while multi-tasking. There is no sense of enjoyment, nourishment, or self-reflection. In this poem, the fact that Halpern gives specific instructions is to make the reader stop and consider why people aren't typically able to stop and take care of themselves like they do for others.
Instead of microwaving, reheating in the oven, or eating cereal, Halpern mixes directions in setting the ambiance and preparing food much like one would expect during a pleasant date or important family dinner.
The last line, "The company is the best you'll ever have" drives home the idea that Halpern is suggesting you need to not only take care of yourself but love the person you are.