Mako was going to kill herself one day.
Not intentionally. Never intentionally. But she had to be EVERYWHERE, save EVERYONE. Last week she sliced her back taking a blow meant for Rei, and before that nearly broke her ankle saving Ami.
Usagi loved that about Mako, loved her selfless devotion.
Understood it, unerringly.
It terrified her.
She harboured a secret fear that every battle would be the last. Mako would do too much to protect her family, forgetting the most important person.
That couldn't be allowed to happen. So Usagi watched, ready at any moment to save Makoto from herself.