It depends on the context of the video being used. If you're taking a video and commentating about it, criticizing it as a work, or mocking it, then that's Fair Use. In fact, there's even a legal precedent for this. The case is called Hosseinzadeh v. Klein. This was a case where Matt Hoss filed a lawsuit against Ethan and Hila Klein of h3h3productions for using his copyright work and mocking it. The judge ruled this was Fair Use, as it transformed the original work. (The purpose of the video was to mock and criticize Matt Hoss in an entertaining way, which has a much different intended purpose of the original work.) This ruling does not apply to all reaction-type videos, however. And has actually listed reaction type videos where there is a facecam in the corner with minimal commentary as something of an exception. So, if you want to do a Laugh You Lose and all you do is stare at the video, you'll probably get copyright claimed. Hope this helped!
