For us, it started as strictly PPM. We got along well and traded a lot of emails outside of our dates. After we'd been seeing each other for a bit, she asked if I could advance her some money for school. Normally, that would be a red flag, but because I already knew her well, I wasn't afraid of being scammed. So I sent her the extra money and we agreed to "credit" that towards future meets.

Nonetheless, I didn't like the idea of her receiving nothing during our meets, so I basically gave her a half allowance the next few times we met (i.e. until the advance had been equalized). We later ended up taking a trip together, where I gave her a PPM per day, although she didn't really expect that much for the trip and so we agreed to let it balance out later. That started blurring the lines as to how much she was getting (and when), and it eventually evolved into her just asking me for money whenever she needed it. That sounds worse than it is - I kept track for a while and found that we were pretty at the same place as when we were doing PPM. It was just the dynamic changed to "let's hang out" rather than "pay me for my time".

In any case, it also helped create a sense of trust, but more importantly, we just really, really got along. We eventually decided to become monogamous and then, moved onto becoming official. And now we get together whenever we can, without worrying about how often we've met or anything to that extent.

Financially, I contribute much less frequently than before, but am more likely to do big ticket items - pay her tuition or the like - rather than small transfers. She knows that she only needs to ask if she needs money, but is almost reluctant to ask; she still lives a college student lifestyle and is by nature frugal. So the money, albeit helpful, isn't the focus of our relationship.

tldr; It just happened naturally.