Part of diagnostic criteria is how long something lasts.

If you have a really good week you aren't going to get diagnosed with mania.

If you have a really good month where you quit your job, move to Italy, and try to start a new job carving marble statues (and you've never carved stone in your life), well those will all be factors in any diagnoses given.

On the other hand, if you feel sad because your dog died, no one should be writing you a script for anything.

But if your dog died and you can't even function enough to go to work for a month, yeah it is probably temporary, but maybe some help is warranted, be that therapy or meds.