The speaker is very melancholic when he thinks of "things past". "I sigh the lack of many a thing I sought" - he grieves for not having fulfilled the wishes and desires of his youth, that has obviously passed. Many of his friends are now dead. But the thought of his "dear friend" changes his attitude by giving him a new sense of fulfilment, that he has otherwise forgotten. It is a tribute to his friend because it is an immense compensation for all things lost in life.
