The history of this is my adolescence dovetailing with the widespread availability of the Internet. Sitting in the blue light from the PC monitor, I'd browse the more adult-oriented chat rooms, read what others were writing to each other, and squeeze my thighs together.
I learned how to give myself an orgasm by finding forums about it online. When I was squeezing my thighs together, I recognized that tight, almost-like-you-have-to-pee feeling as something that could definitely be brought farther than I was. The first time I found the exact pressure from the faucet in the bathtub and had to pull myself back with my legs shaking and my vision over-saturated with light, I was fucking hooked.
There's a similar anticipation now when I'm sexting with somebody. There's a singular thrill to not being able to be with a person physically, but being able to imagine them experiencing the same tightness in their lower abdomen as they browse the photos I'm sending them. I enjoy sexting because, if you do it right, it's like prolonged foreplay. Each photo takes consideration of lighting, positioning, and intent. Waiting on those little message reply dots from the person I'm sexting is a fantastic erotic build-up.