We don’t really know how SSRIs work - or if at all (at least not as intended) - yet they’re handed out indiscriminately for “treating” anything that isn’t “normal”, accepted, or desired behaviour.
These drugs have life-altering - and often permanent - side effects to the extent they’re a case of the cure being worse than the disease for any but the most severe and otherwise untreatable conditions and hence run against basic ethical standards in medicine (“Do no harm.” for one).
Feeding that type of drug to a poor dog, who can neither decide for themselves nor know what’s happening to them, takes a special kind of unethical.
On the other hand, it’s not like the standards around SSRIs for treating humans are ethical either.
Their side effects often are downplayed or kept from patients, who will take those drugs at their doctor’s recommendation, not knowing the full extent of what they have subscribed to.