Do you have to have some type of justified reason to get a restraining order? Or can you issue without reason, on whim, or for fun?
An individual does not have authority to "issue" a restraining order. In the U.S. only judges can issue such orders. The guidelines and policies for obtaining a restraining order vary by jurisdiction, although in the end a judge can issue any order he wants. So in theory you might be able to find a judge who will issue an order for any or no reason. In practice I'd be surprised if many judges would abuse their office to gratuitously harass people in this fashion, though there are probably a few amusing/disturbing examples out there to the contrary.
There are different "flavors" of restraining orders. The most obvious one involves the threat...or reasonable fear thereof...of physical harm. i.e. a domestic abuse case. This type of restraining order is often granted "ex parte" by a judge, solely on the basis of the would-be-victim's uncorroborated testimony. The court wishes to err, if it must err, in the direction of least harm.
Another "flavor" of restraining order is economic: You think another person or party is infringing on your trademark, on your trade secret, on your intellectual property, etc. The temporary remedy is to obtain a restraining order to prevent imminent or ongoing damage, while the machinations of the court process play out. This one has far more grey area and is very unlikely to be granted without some kind of evidentiary hearing that allows both sides to present their case, and the judge will want to receive some analysis of the costs to BOTH parties of granting or denying the injunction.