The Big Bang Theory was on television for 12 glorious seasons. That's a lot of time to cover everything from love to death to comic books multiple times. Over the years, fans enjoyed watching their favorite characters grow and they saw some pretty unusual friendships develop in that time.
Whether it's Raj's relationship with his tiny dog or Sheldon's ability to communicate with his idol Professor Proton, it was these bonds that viewers love to see develop and change. Some characters who started out as enemies even became friends by the time the show ended. These are the relationships that made fans laugh, cry, and even cringe.
Howard, Bernadette, & Stuart
While he was always around as the comic book store guy, Stuart Bloom became a good friend to Bernadette and Howard starting in season 8. After his store burns down, Mrs. Wolowitz allows Stuart to move in with her and Howard. Stuart first befriends her and by proxy, becomes closer with Howard too.
Later, Mrs. Wolowitz passes away and Howard and Bernadette clean out her house and move in. Since Stuart is still living there once the couple has a baby, they allow him to stay as a caregiver. They soon realize that Stuart has been a great friend the whole time. He becomes an unofficial member of the gang and a regular on the show.
Sheldon & Penny
From early on, it was obvious that Sheldon and Penny had great chemistry on screen. Even though Leonard saw Sheldon's quirks as irritating and petty, Penny was much more understanding and treated Sheldon more like a little brother than a potential romantic partner.
Penny and Sheldon couldn't be more polar opposites, but somehow they just click and play off each other's playful banter. Penny constantly teases Sheldon about being nerdy and misunderstood, while Sheldon knowingly makes fun of Penny's job status and personal choices. Fans didn't expect them to work, but Sheldon's overly analytical mind and Penny's good nature work together perfectly.
Howard & Mike (Bernadette's Dad)
Another pair that seem completely opposite are Howard and Mike, played for maximum comedic effect by character actor Casey Sander. He's a gruff retired police officer who cares deeply about Bernadette and doesn't see much in Howard. The two don't start on very good terms, with Mike even suggesting Bernadette "will find another guy."
Things changed for the pair in season six. After being pressured into taking a fishing trip in "The Fish Guts Displacement," Howard and Mike have a heart to heart and both admit that they're only going because their wife told them to go. Mike suggests they go to a casino instead and he'll teach Howard how to play craps. The episode ends with Mike telling Howard he can call him "Mike" instead of "Sir."
Sheldon & Beverly (Leonard's Mom)
As a mother, Dr. Beverly Hofstadter was always distant and professional to Leonard growing up. She's a neuropsychologist, which made her more interested in analyzing Leonard's behaviors than being a loving and warm mother to him. To Sheldon, however, she was much different. Sheldon saw Beverly as the mother he didn't have, and the two shared many things in common.
Throughout the series, Beverly was always open to converse with Sheldon and the two regularly spoke over Zoom, much to Leonard's dismay. Eventually, their relationship turned romantic when Beverly kissed Sheldon while she was drunk. After an apology the following morning, things went back to normal and Beverly and Sheldon stayed friends in the end.
Raj & Cinnamon (His Dog)
Raj never had much luck with women so it made sense that the show gave him a different kind of companion. Cinnamon was his Yorkshire terrier that he spent so much time loving and caring for that it even affected his other relationships and friendships.
In season six, Raj pampers Cinnamon to no end. He walks her in a stroller, makes her gourmet meals, and refuses to put her in a kennel when he has to go to work. By the end of the series, Cinnamon had become a regular on the show and fans accepted that Raj was more loyal to his dog than to many of his potential girlfriends.
Mary (Sheldon's Mom) & The Boys
Mary Cooper is the bible-thumping, straightforward Christian mother of Sheldon. She still lives in Sheldon's home state of Texas, miles away from the gang who are in California. Whenever he feels worried, however, Sheldon tends to return home and that's when Mary's relationship with the rest of the boys is front and center.
Knowing that Sheldon didn't have many friends growing up, Mary is indebted to the gang (especially Leonard) for taking her son in and treating him as a friend. Mary is strict but kind-hearted to the boys as she knows they are always able to set Sheldon straight. Fans didn't expect her to bond with the group due to her religious personality, but her comedic chops made her a perfect fit on the show.
Howard & Sheldon
Howard and Sheldon had a contentious relationship from the start. Sheldon constantly poked fun at Howard for being the only one in the group without a doctoral degree, even though Howard was obviously intelligent because he was an engineer. Howard thought Sheldon lacked social skills, and even though he was no Casanova, Howard seemed to do slightly better with the ladies.
Sheldon always considered Howard as a tertiary friend, but eventually admitted that even he needed a tertiary friend because Howard was the "funny one" of the group. By season 12, Sheldon even helped Bernadette and Howard get out of a pickle by filing paperwork to make the couple's problematic neighbors tear down their deck. In the end, Sheldon chose to help out a friend that was always there for him too.
Penny, Bernadette, & Amy
Penny and Bernadette became friends while working at the Cheesecake Factory, and by season four, the two added Amy to their group. At the time, Bernadette was dating Howard and Amy was a good friend of Sheldon's, but not quite his friend. Since Amy's intellect set her apart from other women, she never really had a group of friends to hang out with before Penny and Amy came along.
Despite their intellectual and career differences, the three often bonded over food and alcohol, trash-talking their significant others and going on adventures together. In the end, both Bernadette and Amy even felt upset that Penny didn't tell them that she was pregnant first.
Sheldon & Professor Proton
Arthur Jeffries (aka Professor Proton) was Sheldon's idol growing up. Sheldon regularly watched his children's science show growing up and even considered the professor his friend since he was the only person he could relate to as a kid. But the two had actually never met in person until season six when Leonard and Sheldon found out they could hire Professor Proton for private events.
Once Jeffries passes away in season 8, he becomes more of a mentor to Sheldon, often appearing to him in dreams in the form of Obi-Wan Kenobi. Professor Proton is there for Sheldon when he's in a bind. Even though the professor doesn't always understand why he's in Sheldon's visions, he always gives Sheldon helpful advice or comforting thoughts that seem to get him through challenges.
Leonard & Sheldon
Sheldon and Leonard are the ultimate friends that fans didn't expect to work out so well. In the beginning, Sheldon interviewed Leonard for a place as his roommate, a step that Leonard thought of as intimidating and unnecessary. While many of Sheldon's quirks got on Leonard's nerves, the two grew closer over the course of 12 seasons.
Over time, Sheldon proved himself to be a good person at heart, saving the gang from an exploding rocket in the elevator and standing up for Howard and Bernadette against their neighbors. Leonard also showed loyalty to Sheldon by retrieving him whenever he ran back home to Texas and helping him navigate his romantic relationship with Amy. Ultimately, the two friends think differently but their characteristics compliment each other.