The obvious reason is, because it's funny. But more on that in a minute.
For those who've never seen it, Big Bang Theory charts the misadventures of four young blokes, staffers of some American university's science faculty. Leonard is the most obviously normal one; Howard wears unfeasibly tight trousers and imagines himself a Lothario; Raj is the Indian one who is too shy to talk to girls (unless drunk); and Sheldon - but where to start? Sheldon is half-human, half-Klingon, the most brilliant, the most self-centred, the most eccentric, effete, dysfunctional and dislikeable.
Howard lives at home with his Jewish mother; Leonard and Sheldon share a flat. Across the hall lives Penny, the blonde wannabe actress with whom Leonard eventually develops an on-off relationship.
Amongst the many likeable things about BBT is the revelation that Penny, for all her lack of education and brains, is in some respects the most intelligent character on display. Whilst the others try and fail with girls, vie with each other intellectual kudos, argue about Star Trek episodes or merely disparage Howard for only being an engineer (the other three are physicists), Penny calmly gets on with her life, working as a waitress between auditions.
Some specific things: first, it is one of the few American TV programmes where the characters look like ordinary people. Actually, quite weird ordinary people, but you get the picture. Even Penny is more girl-next-door than professional blonde.
Two, whilst mocking its characters' nerdiness and pretensions, BBT allows them dignity and humanity in the same way - go on, laugh - Shakespeare does Falstaff. As well as being laughed at, we love them for being the sources of laughter.
Three, the show offers legitimacy for a sort of masculinity - not often celebrated by Hollywood or TV - that is without a trace of machismo. These men may be weedy and ineffectual, but observe their ardour for women!
But back to funny. There's no surer way to kill humour than to try and pin it down, but here is a typical BBT set-up.
Sheldon, Spock's less empathetic nephew, has made friends with a fellow-scientist, the hatchet-faced Amy. In a bar Amy has seen a hunk with whom she feels a powerful physical chemistry. She thinks if she touched him electricity would flow between them. Determined to strike up a conversation with the hunk, she returns to the bar with Sheldon as chaperone. However it turns out the hunk is a brain-dead goon. Disappointed, Amy leaves. As they walk home, she experimentally takes Sheldon's hand. He recoils: "Amy, what are you doing?" She withdraws her hand. "No, thought not", she says.
Did you laugh? No, thought not. Oh well.
I got into BBT because when my wife's away and domestic standards fall, tea is often eaten in front of the TV instead of at the kitchen table. The children look on in amazement as I laugh uproariously at the antics of Leonard, Sheldon and co. "I know why Dad likes it", said one of them the other day. "It's because he's just like them".
