In Pro 27:17, the influence that a man has over his friend is likened to the process of sharpening a knife edge. You see in order to sharpen a knife edge, a sharpening tool made of the same material is used. This tool is then used to rub against the knife edge repeatedly until the edge is sharpened. Over the useful life of that knife this process may be repeated several times in order for the knife to retain its sharpness.
Similar to the process of knife sharpening, the attitude, beliefs, behaviors, etc. of your friend has an impact on your attitude, beliefs, behaviors, etc. A real friend is not a cheerleader. Rather a real friend will support you when needed and will challenge your actions when you veer off course. You must deliberately seek and remain committed to those friendships that enhance you and encourage you to fulfill your purpose. You must resist those friendships that cause you to veer off the path to fulfilling your purpose.
Your purpose is too important to put it at risk because of sentiments or attachments to the wrong kind of friend. Examine your friendships regularly to ensure that you are surrounded by God-ordained friends; friends who will encourage you on the path to fulfilling destiny and not cause you to stumble.