when i select an item it's background is the width of my form but why is my border and background only as wide as what's required for it's content???

Code:
<ListBox HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Foreground="White"  ItemsSource="{Binding}" ItemTemplate="{StaticResource Member}" IsSynchronizedWithCurrentItem="true" ScrollViewer.VerticalScrollBarVisibility="Hidden" ScrollViewer.HorizontalScrollBarVisibility="Hidden" ScrollViewer.CanContentScroll="True" BorderThickness="0" Background="Transparent" x:Name="MembersList"  Grid.Column="1" Grid.Row="1" SelectionMode="Single">

            </ListBox>
Code:
       <DataTemplate x:Key="Member" x:Name="MemberItem">
            <Border HorizontalAlignment="Stretch" Background="White" BorderBrush="Black" BorderThickness="2" CornerRadius="15" Padding="10px" Margin="5px" >
                <Grid Background="Aqua">
                    <Grid.RowDefinitions>
                        <RowDefinition></RowDefinition>
                        <RowDefinition></RowDefinition>
                        <RowDefinition></RowDefinition>
                        <RowDefinition Height="3px"></RowDefinition>
                    </Grid.RowDefinitions>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="30px"></ColumnDefinition>
                        <ColumnDefinition Width="*"></ColumnDefinition>              
                    </Grid.ColumnDefinitions>
                    
                    <TextBlock Grid.Column="1" Grid.Row="0" Margin="2" Foreground="Black" x:Name="Member_Name" Text="{Binding Path=MName}"></TextBlock>
                    <StackPanel Orientation="Horizontal" Grid.Column="1" Grid.Row="1" Margin="2">
                        <TextBlock Foreground="Black" x:Name="Member_ID_Name" Text="Member ID: "></TextBlock>
                        <TextBlock Foreground="Black" x:Name="Member_ID" Text="{Binding Path=MID}"></TextBlock>
                    </StackPanel>
                    
                    <Image Grid.RowSpan="2" Grid.Column="0" Grid.Row="0" Height="30px" Width="30px"  Source="member.gif"></Image>


                </Grid>
            </Border>
        </DataTemplate>