Notice that it is material of the same kind that sharpens; it is not a softer material or a hardened material. This is why most men need another man to sharpen them or they will not reach their finest edge in life. Our wives bring about many wonderful additions and changes to our lives, but some level of sharpening and development must take place within the context of men who are of the same stuff that we are.
This is a fascinating study of context in the book of Proverbs. Proverbs is connected by thought rhymes or conceptual connection. This particular proverb comes right after Solomon has been instructing about the contentious woman. So in some way the contentious woman and men sharpening each other are connected, conceptually, in Solomon's mind.
Let me suggest three different conceptual connections for this proverbial connection.
First, a contentious woman is trying to get her husband to change and yet men rarely change when confronted by a woman. The type of sharpening and improvement that a wife is after is usually most helpful coming within the context of a men's group or one-on-one. I have often suggested that a man get into a serious men's group where men can challenge each other to walk with God, love their wives, be involved at a new level with their kids, etc., and the group breaks through when a wife’s voice has been tuned out. In fact, I have often heard wives say, "Why is he doing what you are saying? I have been saying the same thing for years and he has never listened to me?"
Second, if a wife is contentious, then she believes that some type of change is needed even if she cannot articulately spell it out. It is the wise man who realizes that he may not understand what his wife is trying to get him to do, but some change is needed in his home. Therefore, it would be wise for him to ask for an honest evaluation from a friend or mentor of his for what change he needs to make. Too often men just think that their wives are from a different planet or contentious over nothing because they don't understand what change she is arguing for. A husband would be very perceptive if he just embraced the truth that “If my wife is contending with me, there is some change I can and should make that can bring peace to this situation.” It is not always the exact change she may be interested in and it always has to be boundaried by God's moral boundaries, but there is a change that is needed. Do not blow her off and just think it is her time of the month or a season in her life or she is just too sensitive, etc.
Third, men need to be sharpened by other men and challenged, stretched, pushed, etc. Too many men – after they leave school – leave that tight men's group that they need to reach their full potential. Solomon is saying that a man and his family are not enough for him to be a complete man. We are social creatures and we need other men sharpening us so that we excel to our maximum potential.