Life
Harold meets Maude, and their relationship is all about her teaching him how to live his life to the full as life itself is the greatest gift anyone can have. We also see the reality that life is short and long, a great paradox as we watch Maude die at the end of the film and Harold's long road ahead begin after he drives his car off a cliff.
Bucking Authority
Harold's mother is one that wants him to grow up, take life seriously and get married. Harold, on the other hand, has an obsession with death and at every opportunity he seeks to shock people by pretending to kill himself or others. He even goes so far as to marry Maude, an 80 year old woman he's just met. He is not a man of tradition, but seeks the meaning of life in the way he wants it and no one else.
Friendship
We watch as two seemingly opposite ends of the spectrum people, Harold and Maude, become close friends. They share similar world views and even a sense of humor that to most is despicable. Their friendship leads to marriage despite a very wide age gap, and the worst prank of all turns out to be Maude taking sleeping pills to kill herself after marrying Harold.