By the time of Solomon, iron tools or weapons were the strongest and most effective known in the Biblical world. In historical-archaeological terms, this period of ancient history is even called the “Iron Age.”
Consequently, because of its hardness, “iron sharpens iron” better than any other material in the ancient world. Of course, in doing so, a good bit of heat and even a few sparks gets generated as one sharpens another. Since sharper iron points or blades was desirable, it was a worthwhile effort.
In this verse, that process is compared to the manner in which “one man sharpens another” man’s understanding, attitudes and motivations. We all have the ability and opportunity to sharpen another by our interactions with them.
But those interactions may not always be fun, simple or easy. As we sharpen one another, we will inevitably generate a good bit of heat and quite a few sparks. It’s not always easy, but this process will definitely make us both better.