My take on "reactivity" is that (a) it can be challenging to figure out why, and (b) reactivity has many many variations, as do the causes and solutions!
So about now you're thinking, could you possibly be more vague? LOL!
Some puppies or dogs are reactive in a clearly fearful way. Tail is tucked tight, body is lowered, ears too, body is hunched and lowered, etc.
Others are reactive without ever showing these submissive signs. So "reactivity" can't rightly be lumped into a single category.
I had a rescue that showed all the submissive, fear based reactivity signs, who became one of the most confident, stable dogs I've ever known! So clearly, there's a lot about trying to analyze and understand the "why" that can require an experienced person!
That being said, reactivity for any reason usually requires the same or similar desensitization...time, patience, and clear feedback that isn't angry or over the top of your dog's threshold works if you keep yourself stable, calm, but insistent
Truly fearful dogs can be tougher to help get beyond the problem, but watching and reading their body language is key, keep desensitization sessions for your dog under their threshold, don't press them too far, and many can and will get beyond this...