Please give an analysis of Friar Laurence's speech in act 2, scene 3 (lines 1–30). Which lines are the most interesting?

In act 2, scene 3 of Romeo and Juliet, we see Friar Laurence tending to his garden before Romeo comes in to tell him of the plan to marry Juliet. In his garden, we see Friar Laurence performing a soliloquy that transforms into a monologue when Romeo appears to talk to him. Before Romeo shows up, Friar Laurence is walking around and collecting different plants that act as both poisons and medicines:

For nought so vile that on the earth doth liveBut to the earth some special good doth give;Nor aught so good but, strain'd from that fair use,Revolts from true birth, stumbling on abuse.Virtue itself turns vice, being misapplied,And vice sometime by action dignified.

Friar Laurence is ruminating on the duality of plants and how that same duality can be applied to life. He explains that some plants that we see as poisonous still provide some good to nature, or else they wouldn't be able to exist; and, in humanity, virtue and vice can similarly change to resemble each other. Virtue, when misapplied, can turn to a vice-like love, turning to lust and then sin. Likewise, vice, he explains, could sometimes turn into a good thing if it is used for good, like a man who gambles using his winnings to pay off the debts of others.
This passage is especially important because it foreshadows his plan of marrying Romeo and Juliet. Friar Laurence sees the vice of their parents hate as something he can turn into the good of love through their marriage. Romeo and Juliet are like the "weak flower" possessing the power to do evil and kill each other, but also the power of medicine, to heal the wounds of their families through their union.
Despite his plan, the marriage itself does not bring the Montagues and Capulets together. However, Friar Laurence is right in the end; it isn't the sweet medicine of their marriage that is the vice turning to action dignified; instead, it is their suicides (the vice) that leads to the families making peace (the action dignified). It might not be what he intended, but the truth of his words bear out in the final tragedy of the play.