<ListBox x:Name="listMessage" Margin="5,5,5,5" Grid.Row="1">

    <ListBox.ItemContainerStyle>
        <Style TargetType="ListBoxItem">
            <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
            <Setter Property="HorizontalAlignmen" Value="Stretch"/>
        </Style>
    </ListBox.ItemContainerStyle>

    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <TextBlock DataContext="{Binding}" HorizontalAlignment="Stretch" />
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ListBox>