I'm going to blow out my girlfriend's candle when I leave for work
Why would you blow out her candle?
No reason. I just want to mess with her.
While it seems you are just doing it as a little joke, she may be upset by this. It would be best to just leave the candle alone so she doesn't have to take the time to relight it.
It's my relationship and I can do what I want to.
People are supposed to treat their partners with love and respect though, not do things that may annoy them. At the very least, if you're going to blow out the candle, make sure to relight it for her so she doesn't have to go and do it herself.