I have a recurring motif in my story. It's an object that appears multiple times. The object itself has traditionally had a meaning given to it which is the same as one of my main themes of my story. I thought it would be interesting to place this motif in the story to help emphasize the theme. However, I've been questioned as to what that object means by people who have read the story. Should motifs need an explanation from the author or should they be left up to the reader to interpret? I really, really hate force-feeding my reader with what everything means. I love stories that make people think and come up with their own conclusions about things.
The author is dead. Readers draw their own conclusions about which story elements relate to which themes. In fact, a "theme" is simply a connection the reader (not the author) draws between some ideas. When the author has thought X in his mind and the reader has thought Y in his mind, the measure of the success of the author's creative writing is not the similarity between X and Y, but rather, how much the reader enjoys thinking about Y. Therefore, the author's job is to write something the reader enjoys thinking about, not to get the reader to think what the author is thinking. That is why my answer to your question is: "motifs should be left up to the reader to interpret." Actually, you have no say in the matter. The reader interprets your book however he wants to, regardless of what you say about it. "I've put in so many enigmas and puzzles that [Ulysses] will keep the professors busy for centuries arguing over what I meant, and that's the only way of insuring one's immortality." - James Joyce
Force-feeding readers is definitely a bad idea, but if many people ask you about the meaning of the motif, there might be imbalance in the amount of information they've been given. Be it too little to make sense of it or possibly too much, the motif appearing a lot without an apparent "reason". I'm not saying it's a bad thing. You said yourself that readers should have the option to draw their own conclusions. And it's hard to tell anything without knowing the whole story. If you're unsettled by the number of people who want explanations, then tweaking the amount of related information might be worth a try, but it's pretty tricky.
It may be a case that you know the history of the motif, but it's not as well-known as you believe. In such cases, it instead becomes almost McGuffin-like, in that people are wondering what it means, why it's mentioned, and it never gets explained. Personally, if I was getting that reaction, I would forget the motif. It obviously isn't doing what you thought it would, and explaining could actually take away from the intended significance.
Interesting story about Ray Bradbury regarding Fahrenheit 451. When it came out, almost all readers interpreted the novel as a story regarding censorship and totalitarianism. Ray Bradbury, however, states that it was not about censorship but about illiteracy. Now that we are in the age of reality TV shows coming out in abundance, you'll most likely agree that Fahrenheit 451 is about the drowning out of literature and the force-feeding of depthless TV shows.
We had a similar thread recently where somebody would name a character after somebody they knew (and - in this case - hated) because "everybody knows what a Jason is like". The problem is that not everybody knows the same Jason as you...and even he's probably got friends who actually think he's cool...so instead of it being shorthand for somebody loathsome, it just leaves the reader confused and wondering if you're writing the same language as they're reading. Same with motifs. Things like the swastika, the star of David and the cross are fairly well-known. A well-known symbol may need more explanation if you really need it. And then you need to make sure that your explanation doesn't come across like a dump from Wiki!
Do you have a sense of why they're asking? Do they seem to think it's a flaw? Maybe something you've not resolved properly? Or is it because you've engaged them and they wish to learn more about the world just outside the scope of the story?
Do not over elaborate your underlying message, theme or whatever. They should be like hidden easter eggs. A wonderful surprise if someone discovers it. And if they do not, don't worry. Eventually, someone will come along, find them, talk about them, and then the great easter egg hunt is on!
Writing advice I've seen from time to time in various places: "Resist the urge to explain." In general terms, I think that's good advice.
It's great to allow readers to draw their own conclusion - but there're those that are done cleverly and allow for multiple interpretations and really make people think, and then there're just those who are kinda vague and end up being a sort of "wtf?" So, while it's not good to spoon-feed the reader, and definitley don't force-feed, you do need to give the reader some indication of what you might mean with the object. There's something that's subtly, cleverly implied, and then there's just plain vague. There's a fine line. So, are you sure those readers who've asked what the object means are just clueless, perhaps not your target audience, or is it that your motif needs more explanation?