The euphoria isn't a problem per say, but it will bias the patient in favor of more opioids. Drugs that modify the dopamine system have a tendency to skew opinions in favor of continued use.

For susceptible individuals, these changes can be more or less permanent. The damage was done the minute they filled their first prescription; instead of withholding opioids from those who've already been exposed, it makes more sense to prevent exposure in the first place.

... without meaningful regulations, doctors use these properties to create patients for life. A patient on opioids is much better at scheduling appointments then one on ibuprofen. Given a lack of immediate consequences, even the most well-intentioned doctor is susceptible to large enough sums of cash.