That's because so little context is given. Presumably, randall has an idea about which person's organs are being donated or not. But he doesn't tell us, at least not clearly. He could have shown the whole driver's license, including a picture, so we know if it's the girl or the boy. He could have shown us a little more of a reaction from the girl instead of "blank stare," so we could know if she is generally agreeing with the man or disagreeing. I personally think the first interpretation I listed is correct (which makes the comic only slightly funny, in a sort of profound life-explained-through-toys sort of way) but I really don't know.
So I won't say any more, and maybe the next comic will be simpler.
Prediction: At least one cuddlefish will write a long rant about how dumb i am because obviously this comic is X Interpretation, and I am worthless scum etc and this shows how unqualified I am to write this blog and therefore xkcd is great, but before I get a chance to respond, another cuddlefish will write the exact same post but says it is obviously the exact opposite interpretation.
In Other News: I have just learned of a most extraordinary coincidence. Google Analytics informs me that on September 2nd, 2009, the day of the infamous Comic 631, the number of people who visited my blog after googling "xkcd sucks" jumped up to 631. how can it be??