Whether it was that her sympathy would be so keen, and show itself in so lively a manner, that he was afraid of the reaction upon himself, or whether it was that to his speculative mind all kinds of doubts presented themselves at such a time, pleading and crying aloud to be resolved into certainties, and that he knew she would have shrunk from the expression of any such doubts--nay, from him himself as capable of conceiving them--whatever was the reason, he could unburden himself better to Mr. Thornton than to her of all the thoughts and fancies and fears that had been frost-bound in his brain till now. 
Whether he was afraid of his own reaction to her keen sympathy, or whether he felt she would have shrunk from his doubts and fears at such a time - whatever the reason, he could unburden himself better to Mr. Thornton than to her. 