Why I've got Fatal error of undefined validation function in my form? I get the error in below code, it is strange, why it occurred?

In hook_menu

$items['share/%'] = array(
    'page callback' =&gt; 'drupal_get_form',
    'page arguments' =&gt; array('ym_alter_sharing_content'),
    'access callback' =&gt; 'user_is_logged_in',
    'type' =&gt; MENU_CALLBACK,
);


and:

function ym_alter_sharing_content($form, $form_state) {
  module_load_include('inc', 'node', 'node.pages');
  $nid=arg(1);
  $form = node_add('share');

  //$form['field_nid']['und'][0]['value']['#value']=$nid;
  //$form['#submit'][]='ym_alter_sharing_submit';
  //form_load_include($form_state, 'inc', 'node', 'node.pages');
  return $form;
}


but it return me:


  Fatal error: Call to undefined function node_form_validate()


Where is the problem?
Change page callback form drupal_get_form to function, because if you use drupal_get_form in call back it return form and in your function you create another form, it means form in form and make conflict.

$items['share/%'] = array(
    'page callback' =&gt; 'ym_alter_sharing_content',
    'access callback' =&gt; 'user_is_logged_in',
    'type' =&gt; MENU_CALLBACK,
);