I'm not really a fan of Stephen Sondheim. He has written some good stuff (West Side Story and Funny Thing Happened on the Way to the Forum), but his later work is very much hit or miss.
This is one of his hits. The singer is Dean Jones of Disney fame. The song is "Being Alive" from Company. I loathe Company. But this performance hits a chord. There are better singers than Dean Jones (although he is a fine singer), but this is my favourite version of this song. He owns it.