Humans, being social creatures, do work together to make things easier overall. Some people, whether by innate ability or by circumstance, do have an easier time doing certain tasks, so it is most effective for the common good if each person does what they are best at. If you've ever studied basic economics, this will be familiar. Now, I'm not saying it is reasonable for everyone to act in what the manner would be if everyone worked for the common good. The simple reality is that people won't (and I'm sure it can be argued that they can't) always do that. The possibility of so acting is a necessity. Promoting the common good is selfish in the end - though others will also be selfish in receiving (by not thinking/caring about the cost to you). But it is important to keep each possibility open. --- One way I have said this is "Everybody is always selfish. Some people just have a larger sense of self". That is, your self may cover your family, your tribe, your city, your nation, your planet, or even all life. I'm not saying there is a "right" extent of self for any circumstance. But when you don't ever expand it, that is "too selfish".Disproportionate in relation to what?
What do you see the act of helping others as?