A bit of a darker or at least more introspective question than I usually ask, but I think it's a valid one and something worth considering. It's something I've been grappling with recently myself.
As you reflect on your life, is there something which makes its absence known? Something you wish you had, but don't? Do you think you'll ever be able to change, fill, fix, or work around it? Do you think it'll change with time or remain permanent? How does the missing whatever-it-is impact your life?
A final note: given that people are likely going to be sharing some difficult stuff, it's important to remember that empathic listening, particularly online, isn't usually about offering solutions. Questions are often better than answers in conversations of this type.