I think an important motivator is riffs and exercises that help you find the groove. Maybe that’s what everyone means by “riffs,” but it’s when I am able to ride the groove and keep the beat that makes me think, “yeah, I am a bass player!”
Second is learning songs. I agree that the first time being able to play a whole song beginning to end was a real rush. I found the tabs to a tasteful but super simple bass line (with a nice groove) by Lee Sklar for Blue Bayou by Linda Ronstadt, and played it for a couple of weeks non-stop. It gave me confidence and motivation to dive into playing other, more difficult songs. Another example is Song for my Father, the Horace Silver jazz tune featured in one of the early B2B modules. When I realized that Josh had taught us the whole song, I played along with Horace Silver (7 plus minutes) every day for two weeks.