This is a good opportunity to become familiar with the common communication concept known as "subtext". Most everyone understands that what people say and what they mean are two different things, but all too often become too enmeshed in the superficial context of the argument to explore anything deeper.
An example, the cliche:
A: How do I look in this outfit?
B: You look fine.
A: What do you mean by that?
Comedians love this kind of material because it exposes how communication often leads to misunderstanding. Person A isn't just asking how something looks on them, they're looking for reassurance that they are still attractive to Person B.
Wouldn't it have been nicer if instead the conversation went something like:
A: Do you think I look good in this outfit?
B: Baby, I think you look great in that outfit.
People are people, though, and only infrequently say what they mean. Sometimes it takes a skilled therapist days or weeks or even months to coax the truth out.
In a similar way, until you figure out the subtext of your girlfriend's "silly" question, it's going to be difficult to give her a satisfactory answer. A simple way to get the ball rolling in this direction is to ask:
Why? What are you afraid is going to happen?
There are any number of responses she could give, but suppose she replies something along the lines of:
I just don't want you to forget about me, that's all.
I don't know about you, but I would feel comfortable replying:
I promise I'll never forget about you. Ever.
This can be completely honest. I remember every one of my girlfriends, all the way back to my first kiss. Hopefully it's the reassurance your girlfriend is looking for.
To get back to your original question: There's really no way to avoid silly hypotheticals, at least without becoming a hermit and avoiding other people entirely. Start to pay attention to your own conversation to see how often you say things with some kind of loaded subtext -- you probably do it far more often than you think.
Better to learn to read subtext, and to ask the kind of leading questions that allow you to get to the heart of what others really want or fear.