I call it “regret by the glass.” Even when a friend and I resolve to have just one glass each, we will still often end up ordering another round. And that is exactly the reason wine by the glass sours like last month’s sangria—because when the table is ordering four or more glasses (or, in some cases, just three), it is almost always less expensive to enjoy a whole bottle. Divide the price of a bottle by five, which is the number of five-ounce glasses that a standard bottle contains, and it becomes abundantly clear. Wines by the glass are so marked up that it is practically industry scripture that the cost of the first glass covers what that restaurant paid wholesale for the bottle.
The difference is not only monetary: opting for a bottle over a glass also gives you, of course, much wider choice, since you have the whole list at your fingertips. It also avoids the problem of the wine having gone stale or reds being served too warm, both of which are more likely due to that lonely preopened bottle sitting under the bar.
This is not to suggest that there is no place for by-the-glass selections. Some restaurants offer an exciting diversity of options, especially now that the Coravin device has made it possible to extract wine without having to pull the cork. If you are on an expense account, a few Coravin-caliber wines by the glass could make for a memorable evening.
I, however, prefer to treat wine by the glass like you would a travel-size purchase, confining its use to times of necessity, such as when ordering one or two aperitif or dessert glasses, or when one diner yearns for that extra glass after a bottle is finished. You could also make an argument for ordering by the glass when dining solo, but as you will read, there are good reasons to avoiding resigning yourself to this fate.