Obviously, with all fan theories, you have to take them with a grain of salt. This case is no different. If you're a huge fan of The Big Bang Theory you may either love this theory... or absolutely hate it. It's deep, dark, and a little twisted. But no matter what you think about its validity, it will entertain you. It's off Reddit so you know it's good.
Okay, so let's dive in...
Most people think of Penny, played by Kaley Cuoco, as a pretty girl who doesn't really fit in with the nerdy bunch of guys we've come to love. She falls in love with Leonard, despite the fact that they are very different and she is considered less intelligent than him. Love prevails, yadda-yadda, etc. Right?
Well, the thing is, you kind of come to realize that Penny is slowly losing her mind on the show. From the very start, she is still somewhat normal, but she's freaking out and sad about her ex. She has a lot of infidelity and attachment issues. She mentions at church with Mrs. Cooper that her brother was a drug addict.
She's super polite, albeit frustrated at times by situations that annoy her. She started with a large friend group that slowly dwindled over time. She spent most of her time with her neighbors, who she finds them to be creepy and intolerable, and she has nothing in common with any of them. In fact, she's totally sketched out by Howard. She has a drinking problem and very likely has depression and a tendency for addiction as well.
She ends up spending a lot of time with these guys... like all day, every day. And she's friends with them out of desperation, refusing to get her life together and/or reconnect with her old true friends. She even ends up getting engaged to one of these dudes... Leonard. "Her self-hate and defeating tendencies are dangerous," remarks the Reddit user who posted it, SeraniumFilledClock.
This is not to say that Leonard is free from psychological torment. He was mentally abused as a child by a mean mom, he finds friendship in a roommate who is (more than) somewhat selfish in their relationship. He bullies him. Leonard bases his own self-worth on how much girls like him, since he had no girlfriends in high school. He feels he is inadequate.
"Most of what he does is to make up for what he didn't have." His friends don't support him at all during this difficult period. He gets mocked about the same things that he hates himself for. The reason Leonard and Penny work isn't just because Leonard is smart and Penny is beautiful. They are compatible as they both have self-loathing and sadness within themselves. These problems can drive them both crazy at times. They never face their problems.
This theme of obsessive and semi-neurotic love and relations is the only type of relationship that either character knows how to have. They can't fix each other, but it helps numb or dull the pain. All the characters were abused as children and were left to deal on their own.
Check out the rest of the cast's issues:
Raj's parents spoiled him with material gifts, but never actually took the time to care for him. This leaves him with selective mutism around women.
Sheldon's dad had a temper and was an alcoholic. His siblings beat him up and he was bullied and had no one to talk to about anything that troubled him. He was never taken seriously and turned to his intelligence and hobbies as a way to break free. He's narcissistic, probably has Asperger syndrome, has intimacy issues, a ton of different phobias, all based on the fact that he was never emotionally nurtured.
Howard is clearly emotionally traumatized by his father leaving and by his domineering mother. His obsession with trying to "get" women is an attempt to break out from his mother's grasp. But he can't. Now, in his adult life, everything seems to be a threat to his masculinity and adulthood.
Bernadette was also likely neglected. She mentions that she had to compete with her siblings all the time for attention, and that her mother doesn't talk much. The competitiveness she felt in childhood led to mild sociopathy. She manipulates people from time to time, has sadistic behaviors, intimidates others, acts aggressively, but for the most part flaunts her pretty side because she isn't under threat.
Amy is the most lonely of them all. Her father hasn't come up yet, but she's devoted to Sheldon even though he is pretty cold to her. She has unrealistic expectations from her friends. She was "forever alone" in childhood and has always fantasized about a life with real friends.
And my favorite part. The Tl;Dr
Tl;Dr - everyone (thing) in the big bang theory isn't about nerds, it's a tragedy about people who've lost everything and are laughing it off in a new life together