how rude?

Well I decided to make a second post today.
Something that has been on my mind a lot lately is people's manners. I have been "researching" this story for a long time. One day when I lived in the dorms I held the door open to the drom building for about 10 people all in a row. I noticed how many of them thanked me or just gave me the head nod (which is basically a thanks) verses the number of people that just take advantage and walk through. Now in my eyes it is kind of rude not to thank someone when they hold a door open. I found that day that about half the people said thanks in some form. Of the half that didnt, they didnt even look at me or acknowledge that I was doing something for them.
After that day I just started to pay attension to people's manners. Something I have noticed over the past 2 years is that when there is a group of people, and one person thanks someone for doing something, there are normally a couple other people that will then follow someone's lead and throw in a thanks as well. Overall to me it seems that still only half of the people I see will thank someone for a deed. Most of my experience has been holding doors open, or other little stuff. I would assume for larger deeds that the percentage would be higher.
Something else I have done while at college is thank everyone that holds a door open for me. To tell you the truth I think it is a good conversation starter. One day while cutting though a building while heading to a class there was a cute girl in front of me. There were 4 doors on the path to the next building. She held the first 2 doors open for me (you know the kind of door hold, the push it further open after you walk through). I said "thanks" at the first then "thank you" at the second. Then the next 2 doors came up. She held those as well, so i said "thanks again" and "and one more time". She then said something to me along the lines of "are you stalking me". Anyway long story short, we start talking and have a great conversation. Next door we come up to I said I got this one for a change, and stepped up and held it open for her to walk though first. We had a really good talk, but being the idiot I am, I never got her name or number.
There is something about a simple thank you that feels so good. In a way having someone thank me for something makes me feel like good and satisfied for what I did. It seems though as people either get bored of thanking, or dont feel the need. Like the other day when I bought dessert (food after dinner if i spelled that the wrong way) for a few friends. None of them said thanks, one did offer to pay me for it though. They are friends so I am not mad, but it seems a bit disappointing.
I guess something about the way society is changing bothers me. Too many people live life as take everything for granted, and take as much as you can. I just dont see why people dont take 1 second out of their day to thank someone else for doing something. Just take that one second to hold the door for someone else, or just to thank someone for holding the door. Personally it makes me feel a little better, maybe it will do the same for you.