Maybe it's not an issue of making it formal, per se, but an issue of worrying that if you start messing with a good thing that evolved very organically you'll wreck the natural progression of it? I certainly understand that some things "just happen" and the "don't fix it if it ain't broke" mentality, which I think there may be a touch of here.

If that's the case, roll with it and keep it casual until the time where boundaries that aren't defined become dangerous. Doesn't sound like you're at that point yet.

Best of luck!