In lines 19–36 of "The Lady of Shallot," how does the author develop the theme that living without love leads to sorrow?

In lines 19–36 of "The Lady of Shallot," how does the author develop the theme that living without love leads to sorrow?
The answer to your question would depend on whether you are referring to the 1833 version of the poem or the 1842 version of the poem as well as if your question refers to love or companionship.
Tennyson hints at solitude in both versions of the poem. The Lady of Shallott is "imbowered" on her "silent isle" leading the reader to understand she is imprisoned both physically and socially. "The reaper reaping late and early" hears the Lady of Shallott, only late at night or early in the morning. However, the poem does not mentioned the Lady being heard throughout the day again separating her from the rest of the world and continuing this imagery of solitude. "The little isle is all inrailed...overtrailed." Not only is the island surrounded by a rose covered fence but also overgrown suggesting that the island does not receive visitors. This focus on solitude shows that the Lady of Shallott does not have or look forward to companionship and later in the poem states that due to her solitude and "curse" she lives "with little joy or fear."
The 1842 version of the poem hints even more at solitude for the Lady of Shallott. The poem reads "but who hath seen her wave her hand? Or at the casement seen her stand? Or is she known in all the land? The Lady of Shallott."

The two particular stanzas that you ask about really do not seem to address the theme that living without love leads to sorrow. The first stanza you cite describes a reaper out in the field, reaping barley and listening to the lady's singing. She sings "Like an angel" and the reaper calls her a "'fairy'" as a result of her beautiful, airy voice. The second stanza you cite describes where the lady lives as well as her own, "royally apparelled," appearance.
That being said, the three stanzas immediately following the ones you ask about do seem to begin to address this theme. She has "No time . . . to sport and play" and she must continue weaving or invoke the "curse [that] is on her"; she does not know what the curse is, and so she keeps weaving. "She lives with little joy or fear" and watches the other girls walking around the town in their red cloaks, but she cannot share in their company or happiness. "She hath no loyal knight and true," unlike the "damsels glad" who she sees. Her life, therefore, seems fairly empty because she lacks love.