not giving thanks

One thing that annoys me is when I hold the door for someone (man or woman, well usually the latter) and that person "forgets" to thank me (http://project05952381.blogspot.com/2010/11/giving-thanks.html). These people fail to realize that I've got no obligation to hold any door for anyone. Nonetheless these people think that society (me being part of) owes them something like holding a door and hence don't need to thank me or anyone else for that matter.