If a man does not make new acquaintances as he advances through life, he will soon find himself left alone. A man, sir, should keep his friendship in a constant repair.

This quote means that as a person gets older, it is important to form new relationships and keep the old ones strong. If he does not continue to form new relationships, he will eventually be alone in life. In order to avoid this, he should actively build new friendships and ensure that he maintains the old ones.