Guilt & selfishness. From what I've observed, those seem to be the real reasons why many people here may act selfless on certain occassions.
Firstly, the debate of whether having reasons such as guilt and selfishness are good or bad reasons to be selfless is actually a topic for another post. In that regards, I probably could have chosen a better title for this post, but I wanted it to be catchy. In fact, I will be discussing simply how those are the reasons I've noticed behind some selflessness here and I will not be trying to debate whether those are good or not. The means to an ends thing is a debate that could last 10 posts+.
Secondly, by "certain occasions" I am most specifically referring to dining experiences I've had while here. Similar things about selflessness can probably be extrapolated, but most of my 'evidence' will come from experiences I've had at famly meals.
Now to begin.
It is very common, that when out (or in) at a family (or even friends) breakfast/lunch/dinner, you will see others passing the plate to someone else first. Very forcefully. In a way that goes beyond what you will see in American politeness.
For instance, today my Dad and I had lunch with his friends. When a plate of shrimp came out, my Dad insited his friend have the first take. Where upon his friend insisted, again quite forcefully, that my Dad have the first take. This went back and forth as voices raised until one person was able to outpower the other person (literally in a physical manner) in taking a spoon and shoveling some of it on to the other's plate. This is just one specific instance, but I could go on and on about other occasions of something exactly like this happening (including several more from the same meal). And I guaruntee anyone you ask who lives/lived/visited China will tell you the same.
This would at first seem to be an act of selflessness.
Another example (from the same plate). As the plate of shrimp was being devoured and only a few bits were left, they again fought over who would have the last bit. Again it became a physical fight. I even found myself engaging in such odd rituals with a plate of beef and me giving some of the last bits (with my chopsticks) to my Dad.
This would, also, at first seem to be an act of selflessness.
Now let me explain the motivations for such selflessness. People in China are taught that these are the right things to do. From very young (and I can speak to this from first-hand experience) you are taught to ALWAYS let your 'guests' eat first and NEVER be so greedy as to finish off a plate. And it's grilled and grilled into you. The origins of these ideals are very selfless. In that you should treat guests well, and that you shouldn't be greedy. But because these ideals are forced down your throat, instead of Chinese people doing them because they are simply selfless (as such rituals intend), they do them because if they don't then they feel guilty.
Beyond that conclusion making a lot of sense, I have two further pieces of evidence for this. As I mentioned earlier, I caught myself in the act of following such rituals when deciding to forcefully put some last bits of beef on my Dad's plate. After the fact, I thought about why I did that. I realized it was becuase I felt that I was eating too much of it, and I would feel badly if my Dad hadn't got to try any because of me. So that's straight up some guilt. And it was selfish because I wanted to rid myself of the guilt, and by giving some beef to my Dad I was able to do that. For self-satisfaction.
Another thing is noticing, time after time, people offering something on a plate to someone else, either waiting until they accept the offer or until that person decides to forcefully place the food on the other's plate, and then immediately going in and taking some for themselves. I saw this several times today and will see it more in the future (as I stay here in China). I cannot attest to exactly what these people are thinking when they do this, but I will make an educated guess. They would feel guilty for eating more off that plate, and therefore make someone else take some first (and act as if it was their duty to do so), and relieve that guilt to be able to take some for themselves and enjoy it comfortably.
So these so-called selfless traditions you will find in dining out, and in other situations where courtesy is often extended, have now become just a way for people to rid themselves of guilt. And that is selfish.
Disclaimer though: I do feel like I see some people act truly out of selflessness. And especially when it comes to the older generation (i.e. observing the actions of my grandparents), I feel like when they make courteous gestures they do it because they are just being sefless and really care, rather than for other reasons. And again, I fall victim to this too.