Sometimes, there is something you see (like a movie, a place you visit, another map you see in a videogame...) that makes you want to make a map. It's the spark that triggers your imagination and gets things moving.
While mapping, you might think that your map doesn't really look like what you had imagined, and it's usually frustrating. The interest in the theme of your map can also fade away as other things come to your attention.
My advice, if you are capable of doing a map, and you WANT to make that map, I suggest trying to revive the moment where that "spark" originated.
For example, I would just need to see a Star Trek movie to get me mapping on my minimod again. The last game I was playing was the original Tomb Raider, and consequently the map I'm working on takes place in ancient Egypt.