To make BBPress automatically check the "subscribe" box or "Notify me of follow-up replies via email", place the below code in the WordPress theme's function.php file.
//Subscribe by default
function subscribed_by_default(){
echo '<script type="text/javascript">jQuery("#bbp_topic_subscription").prop("checked","checked");</script>';
}
add_filter('bbp_theme_after_topic_form_subscriptions','subscribed_by_default');
add_filter('bbp_theme_after_reply_form_subscription','subscribed_by_default');