How often have you heard someone say this after they've done something nice for someone else. A selfless act.
But why must we only do one good deed a day?
To me, saying 'that's my good deed done for the day', sounds like you've finished for the day. You've already done something nice, so you'll continue doing what you were doing now. It's all about yourself now.
Why must we keep track of our good deeds? We should be completing so many that it'd be impossible to keep track of.
So, I here by announce you'll never ever hear me say this again. Sure, I'll take notice of my good deeds and feel good about them, but I'm going to attempt to complete as many as I can in one day, every single day. One just isn't enough.