sometimes- it's okay not to understand stuff. My best friend never understands my references or sarcasm. I call her dumb all the time. Infact, she calls herself dumb too but i love her even if she is the dumbest person on this planet and it doesn't even matter if she doesn't understand my sarcasm or my jokes- in the end she'll always understand me when i really need her to and she'll always be there for me and she knows i love her. So- just accept yourself even if you didn't understand your friend's sarcasm- you are not expected to be a genius all the time. Even I don't understand stuff many a times and i just accept it. I am not a robot afterall.