Sweetie Belle calling her Luna instead of Princess Luna

[Shift 4,272]

Sweetie's a kid, she's probably naive to the fact that you should say Princess Luna.

[Scootalove 10,689]

I think that Luna doesn't mind if Sweetie Belle doesn't refer to her as her royal name, besides you don't always have to address someone that way if the person doesn't want to be addressed as such.

[Forlong 1,726]

Because she's a rude little whelp.

[Adorkable 2,737]

I think I understand what you're saying, but I can understand why Sweetie Belle called Princess Luna just "Luna". I think it's more of a maturity and age kind of thing. Kids (or fillies, in this case) can sometimes not completely understand the importance of Princess's titles. Not that they don't respect them, just that they don't really see the Princesses as their rulers, but as their friends more than anything. But that's what I think, anyways. ^^