In my experience there are a few reasons why someone loses that passion:
- that person was never going to stick around anyway
- they have a mistaken idea of the glamor and mysticism in martial arts
- the teacher fails to keep the student inspired
- the student requires a bit of a holiday
For #1, there is nothing you can do. Life is full of people like this who will drift, think that martial arts is for them, then realize that it isn't. That realization may take some time to arrive, i.e. their work life may get too busy, they may have kids, etc., but at some point they re-evaluate what they are currently doing and decide that the martial arts will have to go, it's priority isn't as high as the other things they have got going on. And some people just aren't made for the arts. The trick is to recognize this, and don't waste your time trying to convince them to come back. They might come back some time in the distant future, but in the short term there is nothing you can really do for them, they will come back in their own time if ever.
For #2, you just have to accept this and live with it. Some of these people will stick around for some time before realizing that it isn't really what they wanted, that you don't get awarded a bunch of hot groupies the moment you attain your shodan (in fact the learning gets harder). It may take a real fight before they realize that Jean Claude van Damme's jumping spinning back hook kick that they so assiduously perfected is not really the kick-ass fight finisher that they thought it was. It may be a number of years before they realize that owning a hundred bonsai and practicing wax-on, wax-off isn't going to bring you love, happiness and enlightenment (like it does in Karate Kid (the original one)). If you are a teacher you will get some of these people through your school; teach them what you can but apart from that when they go you need to let them go.
If #3 is the problem (and often it is), then there are ways to fix it. People mature and evolve and learn at different rates. Students can out-grow teachers, especially teachers who have stopped learning themselves. I would suggest a two stage fix for this: first try cross training in another art while retaining your main art, and secondly leave your main art and find a new teacher (or art) altogether.
Cross training in another art can be of enormous benefit to your main art. It can bring new ideas and techniques. It can also help you past some sticking points that the student may have been stagnating on for some time. Not all teachers will appreciate you doing this, some will feel insulted by it, but it is the student's choice and a good teacher should never discourage it.
If you just need to find a new teacher, try and find one in the same art (or in an affiliated school). If possible find somewhere where your previous work can be recognized and credited - if that art still interests you. Otherwise the student will have to dabble in the different arts available to them before deciding on what to follow.
Scenario #4 is also a common one, and frequently manifests around the time the student gets their black belt (or equivalent). They will train relentlessly for a number of years, immersing themselves in the art, and get graded on a frequent basis. Then they reach the magic milestone they always wanted to reach and suddenly they wonder: what's next? It is common for students to either reduce the amount of training or take some time off while they spend some time readjusting their goals. This can be avoided to a certain extent if the school has a good sylabus that is still full of learning after the shodan grade, and a teacher who is accustomed to dealing with students of that grade. If this is the problem then the best thing you can do is be there for the student to talk to, they will need someone to talk with and bounce questions off. They are trying to decide on their next milestone and will often look for affirmation, approval or guidance in their decision making. Apart from being available to the student like this, there is little more you can do because they will decide their next step at their own speed.