Different vibe when on lead?

[MarcoPoloCX]

There's a dog that I walk.
Whenever he sees this other dog off lead, they're fine.
They run and play.
But when the other dog goes on lead, it triggers something in him and he goes for him.
And he doesn't do this to any other on lead dogs.
Do some dogs give off a different vibe when on lead that may cause other dogs to react?

Definitely. One of mine can be like this - great social skills off lead but can really be quite nervous on lead.
From a dog's point of view, the lead prevents a lot of normal dog behaviour e.g instead of polite circling and bum sniffing they are forced to meet face on which can be tricky for some dogs

Being on a lead removes a lot of the dog’s autonomy on how they can deal with things. Basically they’re trapped and couldn’t run away, or show a lot of appeasement behaviour if they wanted/needed to. So for some it instantly puts them on guard, depending on the situation.

Lots of dogs struggle meeting other dogs on lead.

Lots of on-lead greetings are face-on, which can be quite intimidating for dogs. Leads also mean they can't escape if they want and are restricted in their movement and body language.

Mine isn't brilliant at meeting other dogs on lead so we've been doing LOTS of practise and he is getting better. I make him sit and wait with a treat while the other dog goes past. If I know the owner or the say their dog is friendly, we let them meet but only quickly, then he moves on and gets another treat. I want him to see other dogs on-lead as a positive experience for him.

Sadly he was attacked (on lead) by another dog a while ago and it dented his confidence. He ended up with a bite wound on his stomach. That set him back a bit but lots of positive reinforcement seems to work. He even met a JR the other day (the same breed that attacked him) which growled and lunged at him and he just sat and ignored it - I was so proud!

He is fine meeting other on lead dogs but just this one.
Like this morning, they were all playing and chasing.

The other owner puts that dog on the lead and he just went for the other dog.