How does Anne's and Peter's relationship in the secret Annex change?

Initially, Anne felt suspicion and impatience towards Peter Van Daan whom she believes is excessively lazy and self-centered. Because she must share a small space with him and both his and her family members as well as a family friend, their living quarters were cramped and stifling, which only made matters worse.
At first, Peter was irritating to Anne, but as the long months progressed, her attitude toward Peter became more gentle and affectionate. Their relationship changes once Anne notices that Peter has special and tender feelings towards her. In her loneliness, she reciprocates the feelings, but only for a short time; her feelings towards Peter change again when she feels disappointed by him and by his passivity.
Some readers may argue that once Anne actually took the time to get to know Peter, he became more appealing to her, while other readers will argue that Anne had no choice but to see him as an ally of some description. The two adolescents were stuck in a difficult situation, and perhaps survival dictated that they get along and like each other. Anne's loneliness may have been the reason she decided to experiment with a romantic attachment to Peter that ultimately let her down, once she gave him a chance to show her his real self.

Anne initially disliked Peter. However, given their unique living situation, Anne eventually develops feelings for Peter. Since they are both teenagers with no other friends in their age group to talk to, their relationship stems from proximity and convenience.
This book explores the theme of adolescence and how it affects Anne's relationships with those around her. Anne's diary contains thoughts and feelings that would go through a young girl's mind. She desires independence from her parents and to seek affection from the opposite sex.
Despite their inherent differences, it's clear that Anne and Peter do adore each other. Their friendship develops during their time in hiding and sustains after they are found by the Germans. Although the romance doesn't last long, the love that stemmed from dire circumstances eventually blossomed into a close connection.