I think there are two main factors in doing this: The first is creating the passion for the subject matter by making it both fascinating and accessible and by that generating even more curiosity and motivation that I believe are innate in everyone. The second is never letting anyone feel that they are treading water (although this can be seen as a by-product of making things accessible).

Even when babies learn how to walk (assuming a healthy and secure environment) they do not need (or would understand) any assurance that the struggle will be worth it. They go for the struggle because the instinct/motivation to explore is already there - it gives a sense of autonomy, of being in control, of making progress, of being involved, of being alive.

But no need to make assumptions about what motivates babies - you can look at how people feel when playing games, solving puzzles etc. They don't see it as a struggle, it is something that is a joy to participate in, even though it might be very challenging. A great teacher is someone that makes learning a by-product of having fun. I believe this is possible for any subject, no matter how "difficult" or "boring".