Deadbolt, you're absolutely right, that YOU made the choice, but what
hramyaegr is trying to say is that there are genes (and also
memes, which are other agents of natural selection) which will make us more likely to make one choice or another. If all, or most, existing compassionate people have less of a chance of surviving and propogating, then they will not pass on they're gene which caused them to be compassionate, nor will they pass on their compassion to imitators (see
meme).
Obesity is a good example. Suppose that women found severly overweight men very attractive. Women all over the world would search for these overweight men, and try to have sex with them. These men would sleep with at least a thousand women a year. In this scenario, wouldn't you agree that fat men would become more prevalent (assuming obesity is genetic)? But doesn't obesity cause heart disease, etc. which is bad for survival? This is to illustrate that natural selection does not necessarily lead to more desirable traits. The reason natural selection usually does lead to more desirable traits, and why the above example isn't likely to happen, is because there is no reason for women to be especially attracted to fat men, because that will decrease their children's odds of survival, which would diminish the attraction to obese men as a trait in the gene pool. The only way that this scenario could lead to increased survival is if all women were initially attracted to fat men either because it used to increase the liklihood of survival, or because it happened suddenly, in an "outbreak."
Another example are midgets dogs. Most animals that are born as a dwarf are killed very early on. However dogs like chihuahuas or toy poodles have a very high survival rate? Why? Because they're cute. Sure, they may not be the best physical specimen designed for survival, but there are other factors involved too.
Natural Selection is not a magical or mystical force. It is a principle. It applies outside of biology as well.