When do we learn from necessity? We learn from necessity when we want to build or achieve something and to do so we need to learn. E.g. I want to build a calculator, I'll need to learn how basic computation works. Project Based Learning stems out of that need, I want to do X. To do X I must learn Y. I learn Y, then I do X. This is also applied to various job training programs. I want to get X job, to get X job, I must learn Y,therefore I am motivated to learn Y.
When do we learn from curiosity? I guess this can be very familiar to everyone. You're interested in design, you start watching youtube videos, reading magazines, talking to others interested in design. Basically, that curiosity, innate interest, drives all of your learning experiences.
Learning from necessity is usually perverted in eduaction systems through testing. We believe that testing (or for that case graded work) will generate the sufficient motivation for people to learn. But they confuse what people need to do to get a good grade. Learning is not the only way to get a good grade, in fact, it is usually a very inefficient way to get there.
The best learning experiences mix both curiosity and necessity. I want to learn things I'm curious about to do things I'm excited about.
Being motivated to learn is not saying it will be easy to learn. Learning is challenging (like running or reading a long book), but we all know its easier to run on a Sunday morning after a good night sleep and nice weather.