Always say "thankyou" when someone opens the door for you. It's just plain common courtesy. It also makes it worthwhile for that person. Geez, it sounds like you should try holding the door sometime. Then you'd know how it feels not to be acknowledged.

Pah. You're scared of what people think of you. In turn, your actions appear to be rude and selfish to other people. Smile a little more, let your guard drop. People will start to notice you for your good points then.