Do you think it is society that has driven this common courtesy out, or just self-absorbtion?
Perhaps a society of self-absorbtion?
Seriously, I'm not sure that you could demonstrate any one factor as a cause of a social development. Here are a couple of possible factors that strike me immediately.
It has been pointed out that chivalry was always more of an ideal than a practice. This is partly because chivalry was rooted in Christian principles (James 1:27, for instance, says, "Pure religion and undefiled before God and the Father is this, To visit the fatherless and widows in their affliction, and to keep himself unspotted from the world." In other words, one of the key elements in the life of a believer should be helping the weak and disadvantaged.) Unfortunately, the medieval time period was characterized by lip service to Christianity coupled with a complete misunderstanding of or disregard for its tenets (hence, for instance, the atrocities of the Crusades). No surprise that chivalry, rooted in Christian ideals, suffered the same general disregard. Extend this to current society in the U.S. For quite a number of years, the U.S. was, in name and on the surface at least, claiming to espouse Christian ideals. Chivalry also became a part of the society, at least on the surface. In recent decades, Christianity with its ideals and tenets has slipped from the public consciousness. Many think of this as a good thing. But it is perhaps nearly inevitable that something so closely linked to Christianity and its ideals would also suffer a decline. Certainly not all who claim to be Christians have been chivalrous, and not all who are chivalrous claim to be Christians. But I think there is a definite connection.
Second, a key concept of chivalry is putting others ahead of oneself. You put the safety of others (particularly those weaker than yourself) ahead of yourself. You are supposed to put the convenience of others ahead of yourself (e.g. holding doors, giving up seats). (This, incidentally, is another Biblical concept -- Philippians 2:3 "In lowliness of mind let each esteem other better than themselves.") But in our self-esteem oriented society (and here we come to that society of self-absorbtion), we are constantly told, "No one is better than you! You should take care of yourself first." For some of the societal impact of this, give a glance at Lynn Truss' book
Talk to the Hand. Is it any surprise that people now look first to their own safety, comfort, convenience, etc?
For what it's worth, there are still some of us who try to hold to the old manners, or at least adapt them to modern society. I still hold doors for women (in spite of the occasional glare or sarcastic remark), give my bus seat to children, women, the elderly, or disabled, and still get the car door for my wife (when I'm not putting the toddler in her seat, of course). Lest I come across as holier-than-thou, there were some benefits to this -- it sure helped back when I was dating in college, since it made me stand out from the crowd of guys. But I think it's got more to do with the fact that I still espouse Biblical ideals than anything else.