Tension!

The relationship that Virginia Woolf offers between Clarissa and Peter is an interesting one to say the least. The reader learns so much about these two characters' tension through one day described in the novel. They each have flashbacks of more carefree times with one another, the most standout of those being their time in St. James Park.
What makes this relationship almost exciting to follow is the fact that there was (and even still is) an obvious love for one another in the past. They tell themselves that they know each other unconditionally and it is the fact that maybe they know each other so well that in a sense tore them apart. Peter had the small tendencies which annoyed Clarissa and Peter did not appreciate some of the qualities Clarissa had. This is obvious in their present reunion when Peter visits Clarissa at her home. He begins to play with his knife (out of nerve I think- I have the same kind of tendencies when I get nervous too) which immediately makes Clarissa think of the past.
The major thing which turned Clarissa off of Peter is the fact that she knew him well enough to believe that although he had a lot of potential, he never had the drive to move up in society. These two obviously have their likes and dislikes of each other, but it is this way with any relationship. Clarissa, even though it was never actually stated, to some extent regrets how her life turned out after not marrying Peter. She may be in a higher status now than she could have been with Peter, but to gain this she lost something else, love. She is not in love with Richard, whereas Clarissa's differences with Peter should not have been such a factor where she would give up on someone she loved.
In the final pages of the novel Peter attends Clarissa's party and gets alone time with her. He is excited and nervous at the same time, proving that he is in fact still in love with her. Virginia Woolf leaves it at this to let the reader wonder what happened between Clarissa and Peter in her home. No issue is resolved, and although Clarissa is with Richard now, Peter (and Sally) give Clarissa a memory of her past which she misses. It is obvious that the feeling is mutual and they love each other. If I were in the Dalloway household that night, I bet Clarissa would be thinking "what if" with Peter.