While witches or perceived witches were feared and persecuted, they were also shrouded in mystery. Even to this day there are various incomplete theories of how they operate and what instruments they use. However there was also a strong belief that the husband to a witch had nothing to fear because he was essentially the loved one. Hence to the husband she was merely a wife. In this reasoning our elders realized that when some loves something, they will not intentionally harm or spite it.
The proverb is used to explain situations where someone who generally is mean or spiteful does not show these same attributes to a particular person. It can also be used to advise that there is no reason to fear someone who genuinely loves you even though they may be fearful to others.