为什么我不能在WPF中设置所选ListBoxItem的背景颜色？

<ListBox x:Name="MyListBox">
    <ListBoxItem Content="Item 1"/>
    <ListBoxItem Content="Item 2"/>
    <ListBoxItem Content="Item 3"/>
</ListBox>

private void MyListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    ListBoxItem selectedItem = MyListBox.SelectedItem as ListBoxItem;
    if (selectedItem != null)
    {
        selectedItem.Background = Brushes.Yellow;
    }
}

<ListBox x:Name="MyListBox" SelectionChanged="MyListBox_SelectionChanged">
    <ListBoxItem Content="Item 1"/>
    <ListBoxItem Content="Item 2"/>
    <ListBoxItem Content="Item 3"/>
</ListBox>