Why would a man tell a woman she was his best friend?

If a woman tells her a man he is her best friend then its obvious that she doesnt want to lose him but she is hestitant to declare her relationship with him and its just because she is still in search of hunk man for herself or more probably she wants him always by her side but not in her heart and in such cases she shows possesiveness so as not to lose him but still she looks for her new hunk man and search goes on till she is too delicate ..