It was obvious now that he had been remarkably dull; Scott had seen how things were going.
Then he had taken it for granted that Agatha would not find the vein, and had helped her because he thought it better she should convince herself that Strange had been the victim of his imagination.

He had honestly thought this when they started, but now recognized that he had unconsciously had another object: he wanted her society and to earn her gratitude.
A light began to dawn on him when he found Strange's tobacco-box, but he had, so to speak, evaded full illumination until it became obvious that they were near the vein.

Then the truth could no longer be denied.

He was in love with the girl, and had unconsciously loved her from the first.

In a sense, this looked ridiculous; but there it was and he must face it.