How did subject personal pronouns end up being clitics in some languages but proper standalone words in others?
My question is about subject clitics. Recently I learned that in French words like "Je" , "tu" and "il" are subject clitics, they cannot stand on their own, unlike en English, where I, You etc are proper standalone words. How did these pronouns evolve to be clitics? Where they ever independent words at some point in history and start to become more and more dependent on surrounding words? Are there other similar systems to french other than its romance cousins?