Rick and Morty season 5 has brought plenty of new revelations and aspects to the show, and one offers more insight into why Rick is always so especially mean to Morty. With season 5 marking only the first of a potential seven new seasons after the show's most recent renewal, there's no telling where the series will go next. But if one thing is fairly certain, it's that Rick and Morty's eponymous protagonists will probably continue to have one of TV's most toxic, yet occasionally very sweet, dynamics.
Rick and Morty's toxic relationship has been the backbone of the show since its beginning, even getting explicit story treatment in many episodes throughout the series, such as in "Rest and Ricklaxation," "Claw and Hoarder: Special Ricktim's Morty," and "Forgetting Sarick Mortshall." Yet, at the same time, Rick also clearly cares for his grandson on some mostly subconscious, even reluctant, level, as seen later in "Rest and Ricklaxation," when toxic Rick desperately saves Morty or in "Close Rick-Counters of the Rick Kind" when good memories of Morty cause Rick to tear up. Whether for better or worse (usually worse), Rick and Morty are inextricably linked.
Rick has bad relationships with nearly everyone in his life, but none seem as aggressive or significant as his one with Morty. As mean as he is to Jerry, Beth, Summer, and even his possible love interest, Birdperson, Rick reserves a special kind of mistreatment for Morty. This makes sense given that they're the show's main protagonists, but Rick and Morty season 5 also revealed that it could have something to do with Rick's knowledge of how Evil Morty comes to be.
Evil Morty was first introduced in Rick and Morty season 1 episode 10, "Close Rick-Counters of the Rick Kind," and made occasional tease appearances before emerging as the main antagonist at the end of Rick and Morty season 5, in which he escapes with his own portal gun into a version of reality where Rick doesn't control the universe. There's still a lot of mystery surrounding the character, but Evil Morty's Rick and Morty backstory thus far at least reveals that he's way smarter than the average Morty and is driven mainly by a desire to counteract Rick's influence on the universe and find a better life for himself. These key traits set him apart from other Mortys, and could also be the reason that Rick is so much meaner to Morty than to anyone else.
Rick's mistreatment of Morty often boils down to Rick's own nihilism, in which he insists that he doesn't care about Morty, the wellbeing of the universe, or even his own life. In most cases, Rick encourages Morty to adopt a similarly detached and uncaring attitude as well. Likewise, Rick constantly belittles Morty's intelligence and often literally pulls him out of school against his will to go on adventures. Although these actions seem to come mainly from negligence, they could actually have been a targeted effort on Rick's part in Rick and Morty season 5 to prevent Evil Morty's tendencies from developing in his own Morty. If Morty never gets an education or learns to value his own intelligence, or if he thoroughly adopts Rick's nihilism, then he'll never acquire the smarts or the motivations necessary to become Evil Morty.
Thus, in some odd way, it's possible that Rick is meaner to Morty because he cares about what happens to him more than what happens to anyone else. Of course, at the same time, this would still be motivated mainly by self-interest on Rick's part, as Evil Morty poses a threat to his supremacy. Whether or not there's a deeper significance behind it, the central toxic grandfather-grandson relationship in Rick and Morty stayed alive and well throughout season 5, and will likely continue moving forward.