To analyze how an author develops a theme, one needs to look at the literary devices the author used to draw out the theme because it is through these devices that the author also develops the theme. Some literary devices to consider looking for in your analysis are "character development, setting, mood, plot, point of view, figurative language, allegory, symbolism, and irony" ("Identifying Themes and Literary Analysis"). The development of the theme of love should be a fairly easy one to analyze seeing as how the whole play centers around this theme.
The theme of love and its relation to the entire play is first shown in the opening prologue in the lines:
From forth the fatal loins of these two foes
A pair of star-cross'd lovers take their life;
Whose misadventur'd piteous overthrows
Doth, with their death, bury their parents' strife. (First Prologue.5-8)
From these few lines we learn that two lovers falling in love is central to the play, making falling in love a central theme. But we also learn of the plot development that will be centering around these two characters. Namely, we learn that they are "star-cross'd lovers," meaning fated to die and that they are separated due to "misadventures," meaning mishaps. Most importantly, we learn that the couple's death puts an end to their parents' feud. Hence, we learn that plot development is one means through which Shakespeare developed the central theme of love, through showing us a contrast between love and hatred and even putting an end to hatred via love.
We can also learn about the development of the theme of love through looking at the recurring motifs. A recurring motif is any element that is repeatedly used in a literary work. A recurring motif can be a literary device, a word, a symbol, an action, or many other things as well. One recurring motif we see in relation to the love theme is the recurring image of fire. When Romeo first sees Juliet, he compares her beauty to fire, saying, "She doth teach the torches to burn bright!" (I.v.46). This image also relates the love he feels for her to a burning fire. But more importantly, Friar Laurence makes the philosophical point that the type of sudden, intense, passionate love Romeo and Juliet feel for each other often comes to a very sudden end, just like fire being quickly quenched by powder, as we see in his lines, "These violent delights have violent ends / And in their triumph die, like fire and powder" (II.vi.9-10). Hence, this one recurring motif of fire shows how Shakespeare developed the theme of love to show that the type of passionate, impulsive love that Romeo and Juliet felt is not the strongest, purest, longest lasting sort of love.