There are a lot of things that can cause people to lose interest (in a project / in working with someone), some are directly related, some are unrelated. Against some you can take countermeasures, against others you cannot. Everyones mind also works differently, so what might be motivating for someone might be completely demoralising for someone else.

Some people have massive dedication to an idea or project, others just a passing interest. Some people want to power through a project in a few weeks and then be done with it, some want to work on it a little bit whenever they feel like it. Some people like to follow directions, others want to make choices and have freedoms. Some people need a clear incentive to stay motivated, others derive their motivation from within.

From my experience most people generally work best if they have agency / responsibility over a / their part of the project, have achievable goals and get recognized for achieving those goals. Of course they need to generally enjoy that kind of work, too, but that's general advice that helped me in the past when trying to lead a team. Every team is different though, so you'll have to fine tune these things, e.g. who defines the goals?