Someone can need reassurance without needing you to make a promise you cannot keep.

Fear asks for certainty, and the pressure to supply it is enormous, especially when you love the person. So promises get made about outcomes nobody controls: it will be fine, they will be fine, nothing will change. The relief lasts until the first contradiction, and then the reassurance itself becomes something they cannot rely on.

Say what you can honestly say: that you are here, that you will not disappear, that you will find out together. Let the unknown parts stay unknown out loud. That kind of comfort holds up later.