Why are my checkboxes unchecking when changing Page? WPF C# .NET

So basically.. I added 2 new pages in a frame and when I press a button it changes frame..

But if I check a checkbox on the first page it wont be checked if I go to another page and then go back to the first one..

here is a more visual look http://ift.tt/2cHlhsB

Source code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace CheckboxesAndPages
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void button_Click(object sender, RoutedEventArgs e)
        {
            main.Content = new SecondPage();
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            main.Content = new thirdPage();
        }
    }
}

There is no source code for the 2 pages.