just the absurdity of donald duck putting out that statement (which even makes some sense and fits donald’s overall depressed mood) and mickey mouse being the sly one and contradicting donald, coming out with the upper hand from the argument, is just so amusing.
no, the joke is that you don't actually know that the chemicals inside your brain are actually chemicals. for all we know, we could all be living in a big computer simulation, and your feelings are actually caused by bits and bytes on a computer. so, we only think that we're driving on a brain powered by chemicals because of the things that we can observe from within the simulation, in other words, the simulation makes us believe that we're only feeling love because of silly chemicals doing things. meanwhile our feelings could be much more universal and come from the simulation itself (which could itself be universal), while the chemicals are only a pretense to make the simulation more believable.