Accumulating of individual and universal sensibility in a person with an exiting power can be described with the word “heart”*. Heart embraces lots of meanings in our mind.
Therefore, when this accumulated sense leads the person to act in a problem solving way without waiting anything in return, this can be called voluntariness.
The opposite of voluntariness can be mentioned about the things that are being forced to do unwillingly.
In a situation where there is no force or no calculations, willing to do something uninvited or willing to be in a group that does such things again uninvited, then this is to be volunteer.
When there is no obligation and no force, there is nothing in return and it is not demanded.
It is said there is nothing in return but there has to be and there is. Such as general kindness. Such as kindness that can be shared on a personal level.