All the jokes sound the same and nothing's funny anymore. 

"People often fall out of love when they are depressed because they no longer feel connected to themselves and their partner," says licensed therapist Abbey Sangmeister, MSEd, LPC, ACS. "Depression creates a fog around us that does not allow us to see or feel clearly, which can cause us to feel that no one loves us, we feel or don’t have the energy to love and give, or feel overall numb and disconnected."

Could this be why I feel unfulfilled in this relationship?

Maybe I'm just depressed.

I think this is good enough reassurance for now.