In the day to day of our relationships, a lot of stuff can get swept under the rug.
Combine that with the fact that a lot of partners don’t really get to know each other on a deep level at the beginning of their relationships (or at any point) and you could be highly prone to emotionally stepping on your partner’s toes without knowing it.
This may be a difficult question to ask, but it's often a great one to give you insight into someone's personality.
Asking someone about the hardest experience they have ever faced will let you know how they feel about adversity, as well as how they overcame that adversity.
It'll also tell you what they consider difficult, on an interpersonal level.