The truth is, there’s never one good, concise answer to this very personal question. In most cases, the decision to separate didn’t come about over any one single issue but rather several problems that built up over time. If you’re asking because you feel you’re in the same bind in your own relationship, try relating instead to see if they open up to tell you more. Try: “Hey, my husband and I haven’t been getting along and I’m wondering if I’m in the same situation as you.” See if they tell you more; if they do, it will be clear that they don’t mind discussing it. If not, let it go.