The main characters, Henry and Clare, each give a short intro to the nature of their relationship.
For Clare, the hardest thing about being a time traveler's wife is the waiting and her longing for Henry when he's gone.
For Henry, it's his lack of control over when and where he goes. Sometimes the way time grips him feels euphoric, but mostly it's a difficult quest for survival, when he pops out in a foreign time, naked, with no food and shelter, and no way to explain his situation. "When I'm out there in time I'm inverted, changed into a desperate version of myself" (Prologue.15).
What he really wishes for are ordinary days and just being with Clare: "I hate to be where she is not, when she is not. And yet, I am always going, and she cannot follow" (Prologue.15).