Service to mankind is something that many of us are encouraged to do. I would argue that at times, we are driven by ourselves to work for the benefit of others... but what is true service?
When I think of service, I think of selflessness. I think of working for the benefit of others, regardless of the net benefit or gain that you receive.
This, however also means that the principle of service supercedes the audience receiving that service... what does this mean? It means that even when one hates the individuals one is expected to help, even if the crowd one is serving is bigger or smaller than expected, even if it means that you have to deal with a situation that is unpleasant or unwanted... the true principles behind serving others needs to be foremost in the mind of the individual who wishes to serve.
The principles behind service lie in the root of being a human being. As a human, we are all gifted with the ability to be altruistic. We can work purely for the benefit of others, not for any personal gain at all. I believe that this principle plays an important role in what we view as service. Other entities in creation do not often work for the benefit of others if it is detrimental to them... however, as human beings, as creatures that have a spark of intellect, we are able to work for others, even when that hurts us.
Service also brings with it the basic principle of humility. When we serve others, we serve God... we serve God as He appears in that other individual... what do I mean by this? Service to mankind, is truly service to God. When we see an individual who is struggling, when we see someone that needs help, most people want to help that needy person. I would propose that the desire to help another, particularly in their time of need, may derive it's impetus from evolutionary grounds (working in a joint social environment does drive that sort of behavior), however, the lengths to which we can go to help another in need can be astounding. People can sell all of their belongings, give away everything - simply to help strangers.
What could this be, but humility? Knowledge that we live as simple creatures, and we will die as simple creatures. The only thing that we can put on this Earth that will last, are our actions and deeds.
Service, in this way, is the truest expression of humility there is, for when we serve, we are bowing ourselves to God, because we see God in everything and everyone we serve.