New relationships flood the brain with dopamine. The chemistry can't last—and it isn't supposed to. The plateau that follows isn't the relationship failing; it's the relationship becoming sustainable.
An object in motion stays in motion until acted on by an external force. Relationships work the same way—it's far easier to keep one moving than to restart one that's come to rest.
Every relationship has an entropy coefficient that drifts with time. When it runs parallel to the time axis—or perpendicular to it—the relationship ceases to exist.