Adam Clarke on Proverbs 6:1

If thou be surety for thy friend - לרעך lereacha, for thy neighbor; i.e., any person. If thou pledge thyself in behalf of another, thou takest the burden off him, and placest it on thine own shoulders; and when he knows he has got one to stand between him and the demands of law and justice, he will feel little responsibility; his spirit of exertion will become crippled, and listlessness as to the event will be the consequence. His own character will suffer little; his property nothing, for his friend bears all the burden: and perhaps the very person for whom he bore this burden treats him with neglect; and, lest the restoration of the pledge should be required, will avoid both the sight and presence of his friend. Give what thou canst; but, except in extreme cases, be surety for no man. Striking or shaking hands when the mouth had once made the promise, was considered as the ratification of the engagement; and thus the man became ensnared with the words of his mouth.