Because she hadn't had a real body in ages (a long time) and wanted to experience a kiss, plus thought the Doctor was hot even though she didn't particularly like him, and its very possible that she could also feel Rose's own feelings toward the Doctor and that helped her to act on it.
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Rose and the clone Doctor kiss in Journey's End (series 4 episode 13). Although Rose and the real Doctor have kissed, Rose was possessed by Cassandra at the time.
Zoë Wanamaker.
I cant find it
Cassandra Compton is 5' 5".
Cassandra Gava's birth name is Cassandra Gaviola.