Most Helpful Guy
With time and patience. Sometimes it can happen fast or slow.1
Most Helpful Girl
From my experience, it just happens automatically.3
I just carry on dating until the girl asks to be exclusive. I also never say "I love you" first.
Ehh, here in the UK, we don't really have dating. For us, we just ask someone out (which doesn't necessarily mean a date), and we're kinda already significant others then. Casual dating doesn't seem to be what we do very often. But I guess the best way to transition from dating to significant others would be to officially ask them.
I would think one would officially ask the other to be their boy/girlfriend to transition into more than just dating.
It happens naturally when you don't break up, I suppose.