I think he's talking about the fact that an object doesn't suffer from the sadness that comes with being a human. It also doesn't have the ability to judge, (i.e. "But you don't know what I go through"). The knife will never be affected by feeling or emotion and won't be mentally battered by the passage of time, even if it obviously will physically ("It will not rust through the tears, And it will not lose its appeal over years").
I think he's talking about the fact that an object doesn't suffer from the sadness that comes with being a human. It also doesn't have the ability to judge, (i.e. "But you don't know what I go through"). The knife will never be affected by feeling or emotion and won't be mentally battered by the passage of time, even if it obviously will physically ("It will not rust through the tears, And it will not lose its appeal over years").