Believable is important because you have to internally 100% without a doubt believe that what you're doing is the right thing to be doing now.
As soon as the "what ifs" starts to creep in for the big picture items or goals, that can destroy everything. I'm not talking about running into technical implementation problems along the way (those can be fun), it's more like "did I pick the right language for this?" level of questions that sit in the back of your mind.
Personally when I find something to work on that I like and will have what I think is a favorable outcome, it's easy to put in 8-10 real 100% laser focused hours into a task every day, even if it spans weeks or months. I'd like to think most people can do this too, the hard part (for me at least) is having these things to work on.