She was evidently very uneasy about it.

"I don't see why you think because a young man and girl are in the same house anything like that is bound to happen," said she.
"Well, perhaps not; maybe it won't," said Henry, soothingly.

He saw that it troubled Sylvia, and it had always been an unwritten maxim with him that Sylvia should not be troubled if it could be helped.

He knew that he himself was about to trouble her, and why should she be vexed, in addition, about an uncertainty, as possibly this incipient love-affair might be.

After all, why should it follow that because a young man and a girl lived in the same house they should immediately fall in love?
And why should it not be entirely possible that they might have a little love-making without any serious consequences?
Horace had presumably paid a little attention to girls before, and it was very probable that Rose had received attention.