Why is Git not allowing me to fast forward merge anymore?

If I try to force it using --ff-only, I get the message

fatal: Not possible to fast-forward, aborting.

I realize that there are huge advantages to merge --no-ff, but I'm just puzzled why I can't --ff-only now?

Disclaimer: these commands will bring changes from the remote branch into yours.

git pull --rebase. Unlike the other solution, you don't need to know the name of your destination branch.

If your upstream branch is not set, try git pull origin <branch> --rebase (credit to @Rick in the comments)

To set this option globally, use git config --global pull.rebase true (credit to @Artur Mustafin below)