Custom Session Values Disappears in WordPress

I am trying to keep $_POST values in $_SESSION, so I can keep pagination working for a specific search. I have code below:

if($_POST){
	foreach($_POST as $key => $value){
		if($value != ''){
			$item['taxonomy'] = htmlspecialchars($key);
			$item['terms'] = htmlspecialchars($value);
			$item['field'] = 'slug';
			$list[] = $item;
		}
	}
	$_SESSION['realty-search'] = $list;//!empty($list) ? $list : array();
}

$search = !empty($_SESSION['realty-search']) ? $_SESSION['realty-search'] : array();

$cleanArray = array_merge(array('relation' => 'AND'), $search);
$args['post_type'] = 'listings';
$args['showposts'] = 3;
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
$args['paged'] = $paged;
$args['tax_query'] = $cleanArray;

I do a print_r on $_SESSION – and it works when form submits. But as soon as I go in another page – $_SESSION disappears? Is there a specific way I should be handling sessions in Wordrpess?