In chapter nine, Watson relates witnessing an altercation between Stapleton and Sir Henry. Watson observed it from a distance, and couldn’t hear what the men said. The interaction appeared to have been very heated:

He was running wildly towards them, his absurd net dangling behind him. He gesticulated and almost danced with excitement in front of the lovers. What the scene meant I could not imagine, but it seemed to me that Stapleton was abusing Sir Henry, who offered explanations, which became more angry as the other refused to accept them.

Sir Henry would later confirm that Stapleton had been "white with rage."

During the encounter, Stapleton must have realized his behavior looked odd, because he explained himself to Sir Henry, and Sir Henry later shared this account with Watson.

According to Sir Henry, Stapleton said "his sister is everything in his life." He has been "a very lonely man with only her as a companion, so that the thought of losing her was really terrible to him." When Stapleton saw his sister with Sir Henry, he felt threatened at the prospect of losing her (to marriage with Sir Henry). He hadn't realized there was a romantic attachment forming, and so when Stapleton "saw with his own eyes that it was really so, and that she might be taken away from him, it gave him such a shock that for a time he was not responsible for what he said or did."

In other words, Stapleton claims he overreacted—and behaved impulsively—because he viewed Sir Henry as someone who would take away his companion and helpmate.

Sir Henry clearly thinks this explanation is inadequate. He can't understand why Stapleton would react so vehemently to him as a potential brother-in-law. Sir Henry has wealth and a title, so Stapleton "can’t object to [Sir Henry's] worldly position."

Thus, Stapleton's explanation leaves unanswered questions, and deepens the mystery.