Everyone who owns a dog should have a basic understanding of dog behaviour which includes how dogs become reactive and what aggression is... and isn't.
If you can't be arsed with that OP - please rehome your dog.
Dogs become reactive for a variety of reasons, a classic one is the dog on a lead, has a nasty experience with another dog (or a person, or traffic...) and that event or series of events, plus the feeling of being trapped by the lead, denied the option to run away, express normal body language, hide, freeze, fiddle about... means they feel they've no option left but to resort to lunging/growling/snapping..
Some of these dogs are bluffing and if the threat level increased ie, the trigger for the behaviour got nearer, they'd drop to the floor or try to bolt.
Some will not be bluffing and would genuinely bite in order to protect themselves.
If you see someone with a dog on a lead, give them a wide berth - there is NO need for your dog to meet other dogs on lead. Focus on teaching your dog to associate the sight of other dogs at a distance, with high value food rewards or toys (whatever the dog finds reinforcing), no matter what the other dog is doing, as long as it is safe to do so.
The more you can do this, the more you're proofing your puppy against a negative experience, but there is no guarantee, because as soon as you set foot outside your home, you are in an environment you can't control.
You could meet a friendly but rude dog next week who bounces on your puppy after a polite sniff, or bounces too NEAR your puppy before the polite sniff, scares your puppy, hurts them accidentally... and your puppy decides its probably safer to pretend to be a savage beast to get the space he now feels he needs, when he sees another dog.
Secure fields are a great thing - however they do need booking in advance, and may not be an option for people who do not drive or whose dogs cannot go in the car.
I'd look for some though - if you carry on without any understanding of dog behaviour, nor any desire to learn, you're going to be needing a secure dog field for your own dog soon enough.