Why is it so hard to say Thank You?

Why is it so hard for us to say 'Thank You' to the people who help us?  Not just the quick “thanks” for the people who hold the door open for you, but the “going out of your way thank you” for the people who go above and beyond to help us or do something special.
If we have a problem with someone, we can be vocal, sometimes very vocal about it - especially making sure everyone around knows about it.  But too many people seem to just let it slide by if we get something nice.  
Having a fairly decent camera in my hands at an event, therefore taking a fair number of good pictures, I am more than happy to pass them on to a diving club, athlete/coach/family, school etc. They get some good quality pictures as a 'Thank You' from me for their efforts at the event.  While it is always nice to get an email or phone call to say 'Thank You' from the recipient, I am puzzled by the level of people who don't bother to provide any response.  Were the pictures pants?  Were they not what they thought?  Could they just not be bothered?
Anyway, moral of the story - don't bother worrying about the people who don't know how to say 'Thank You', just make sure that when you get something good, whether it be a service or something tangible, try to say 'Thank You' - it does mean a lot to the giver.