One major theme of this poem is that people tend to glorify and idealize war, but they are ultimately unprepared for the devastation it causes. For example, the lovely girls who used to dance with the soldier—who is now legless and missing an arm from war— "touch him like some queer disease." Before, they found him so attractive, and now they want nothing to do with him. Further, he used to be young and handsome, so much so that "an artist [was] silly for his face" and someone had told him he would look like "a god in kilts," but now his face has lost its color and "he is old." He finds himself alone. Though "he was drafted out with drums and cheers," when he returns from war in pieces, only "some" come out to welcome him and just one "solemn man [...] / Thanked him." The gratitude people seemed to feel toward the young man before he left turns to horror at what has become of him.
Even "his Meg," the girl he wanted to please, for whom he joined the military in the first place, is never mentioned now. It seems that she is no longer a part of his life. He was made to believe that he would be a hero, but now he is the object of "pity," and he watches as "women's eyes / Passed from him to the strong men that were whole." He is tired now, and forgotten; he is shut up in some institute, presumably for the "disabled." Thus, we see how different a life he has from the one he imagined before the war. He was allowed to idealize it, never imagining the horrific effects it could have on him, mentally and physically. Nor were others, apparently, prepared for these effects, and now they have a hard time looking at the soldier because he is a terrible reminder of their own naivety: this naivety is something they would rather maintain because it is easier to think of war as glorious rather than tragic.
In Owens' work "Disabled" the recurring theme seems to be two-fold: Loss and Disillusionment.
When he was young he looked at the world as a young man always does - unrealistically. The attitude he has is the embodiment of what Psychology calls "The Personal Fable" by which all young people live by, which says, "We will live forever, and the whole world revolves around us."
He thinks of war as glorious and exciting. The assumption is there that the uniform and all that it represents will impress all who see it, especially young ladies. And that it does at first, before any shots have been fired or wounds suffered. Even artists want to paint his portrait - before.
But once he returns for real battle, wounded and fatigued, everything has change. Ironically, when he "deserves" attention and admiration that is the farthest thing from people's minds. They flee his presence. They resent the feelings the sight of him evokes.
So he sits in his chair, having lost such a great portion of himself and disillusioned at how the would should have treated him.