What is straight? A line can be straight, or a street, but the human heart, oh, no, it's curved like a road through mountains.

The speaker contrasts the geometric certainty of “straightness” in objects (a line, a street) with the emotional and moral complexity of human beings. The “human heart” is figured as inherently indirect—shaped by desire, fear, memory, and contradiction—so it cannot be expected to move in clean, predictable paths. The simile “curved like a road through mountains” suggests both inevitability and necessity: terrain (circumstance, inner conflict) forces turns, detours, and switchbacks. Read this way, the quote resists simplistic judgments about sincerity or virtue, implying that what looks like inconsistency may be the natural form of human feeling.