"The Ruined Cottage" is one of Wordsworth's bleakest poems. The speaker encounters an old man near the ruined cottage. He narrates a tale about a woman who faces hardship her entire life. Some of her troubles include poor harvests, harsh winters, the death of a child, and losing her husband in war. The physical deterioration of the cottage, over time, parallels the deterioration of this woman's (Margaret's) life over that time. That being said, one of the themes is human suffering. Carrying that theme into other philosophical tangents, we could say that this poem is about trying to be hopeful in hopeless situations. The old man/narrator continually tries to comfort Margaret with only temporary success. Hoping against hopelessness and dealing with suffering are more specific themes in this poem.
A lot of Wordsworth's poetry is nostalgic, longing for more natural and creative experiences which are often associated with youth. He, like Keats, also addresses the fear of death; all the more reason to fight fervently for genuine and authentic experiences, and all the more reason to wonder about, and battle, human suffering. This is one of the reasons Wordsworth wrote about common and rural life. Margaret lived a common and rural life during which she endured more than her fair share of suffering. Her reluctance to leave the deteriorating cottage reflects her genuine and simple perseverance in a hopeless situation.
She loved this wretched spot, nor would for worlds
Have parted hence; and still that length of road
And this rude bench one torturing hope endeared,
Fast rooted at her heart, and here, my friend,
In sickness she remained, and here she died,
Last human tenant of these ruined walls. (485-92)
The phrase that best captures this idea of hoping against hopelessness is "one torturing hope endeared" because this indicates a paradoxically statement: hope can be inspiring but in the bleakest situations, hope tantalizes (teases) but never emerges.