Why does my dig get aggressive when on the lead?

[mckenzie]

We have a 20 month old Tibetan terrier who is very chilled at home, friendly and playful when off the lead but sometimes very aggressive towards other dogs when he/they are on the lead.
Any ideas why please and any tips for changing the behaviour?
TIA

This is very common. Dogs on leads are not free to engage in natural greeting interactions with other dogs, which, significantly, include the freedom to walk away from the other dog. Feeling restricted and threatened, dogs try to defend themselves.

There is some good information here
positively.com/dog-behavior/aggression/leash-aggression/

It's well worth dealing with this before it escalates as when this becomes an entrenched behaviour it can severely limit where you can take your dog.

Thank you booboostwoo
It's rather a relief to know it's not unusually and I totally get the fact that I could be making it worse as I do tense every time we approach another dog.
I shall follow the tips given and see how we get on.

It's difficult not to tense up when you know your dog will kick off. Try whistling or singing - I kid you not, it regulates your breath and takes your mind off things. I use singing a lot with nervous horses!