I think that Yeats' poem discusses the complex nature of being brought about by the death that is such an intrinsic part of war. For Yeats, the poem reflects this at the point where the speaker recognizes that there is an end to life and it is one that is rapidly approaching for him. The pain of life is evident in that the speaker recognizes the finite nature of being. His life is going to end and the poem indicates that there is nothing to be done about changing this end. The pain that is there is evident in the reasons why the speaker is not fighting, almost indicating a poignant aloneness that is evoked in his condition. All soldiers, the poem suggests, might be a part of a collective entity, but they face death alone. There is a condition of pain in such a reality. If there is a joy present, it might be explained in the reason why the airman joined the fight in the first place:
A lonely impulse of delight
Drove to this tumult in the clouds;
The "tumult" where pain is evident was initiated by a "lonely impulse," a love of flying that is shared by few. It is here, in the love of what one does, where there is some level of joy evident. The ability that the airman will die is painful, but it is one in which the airman dies doing what is loved, something of "a lonely impulse of delight."