Because she still loves him , but they spend too much time apart, Samantha and Smith live in L.A.in the movie and he's always working.
She makes and effort to do things, such as, putting sushi all over her naked body on valentines day but he comes home late and its all ruined. She hears the neighbour having sex everynight, (something she isnt getting) and figures lifes too short
she says something like 'I love you , but I love me more'