Why do dogs behave differently when on the lead?

Rupert can get a bit growl-y if he is on lead and we pass another dog on the way home on the (busy) pavement.

I don't understand why dogs behave differently while on the lead.

Can anyone explain?

I've always read that it's due to feeling trapped in lead and unable to escape/defend themselves. A lead attached to a collar/head collar can change the natural body language of a dog and can crate some odd signals that they dog isn't meaning to put across also. When walking past a dog as close proximity on lead a dog's movements are much more ridged and less fluid. But I think it's mostly due to feeling stuck in one place.

That would make sense   

Darcy can be a bit of a cow on lead? Not all the time just occasionally I put it down to feeling trapped.

I would agree being on the lead can make a dog feel like they have no way out as such so the only option for them then is to go through whatever is there

Also if you are feeling a bit nervous and because of past experiences of him growling you are thinking he will do it again so he will feed that nervousness of you and that can trigger a reaction.

my sam is differnet also he doesn't like big dog ( which he only started when he was attack by 2 huge boxers) and he does act a bit dodgy with lads with hoodies which i dont know why & dont mind

Moo is a pain on lead, off totally perfect but on can be slightly temperamental. Fortunately she isn't on it very often Xx

They certainly feel tension from the owner through the lead.. Having witnessed a very aggressive sounding Staff on a lead whilst my Flo took no noticed and walk past whilst off the lead. The owners were quiet obviously responsible which was good but you could see their concern.

Flo used to pull on the lead but having built trust between us off the lead, she is now a completely different girl and walks lovely when on the lead.

Granted this doesn't work for all though