I'm not taking anyone's side in this discussion, but I think people forget that Charles was also under tremendous pressure to marry at the time he reached 30, from the royal family and from the media. There were lots of newspaper articles saying that he should get wed, and public speculation about possible brides, including foreign princesses.
Various 'suitable' young women had, by reliable accounts, refused him. Camilla was married to APB, so no longer available. And in any case, she was not considered suitable by the royal family or its advisors.
At the same time, as a previous poster said, Diana was put under pressure by her family, especially by her grandmother who was very close to Charles's grandmother. Also, her father was a childhood friend of the Queen, served the Queen and her father King George as an equerry , remained close to the royal family and was proud to have his daughter marry the Prince of Wales. Diana's mother had married at 18. Diana did not have a career, but a series of 'little jobs', just like so many young women of her background who were in essence waiting around until they found a husband.
Neither Charles nor Diana had any experience of 'normal' family relationships. I think Charles meant it when he famously said at the time the engagement was announced, 'whatever love means'.
Last but not least, in 1981- it seems amazing to think that Charles and Diana's wedding was 40 years ago next year - attitudes really were not the same as they are today, especially not for people like Charles and Diana. Attitudes were changing, but there was still real and unresolved tension between ideas such as marrying 'for duty' or because 'it was expected' and ideas such as marrying 'for self-fulfilment' or just because you wanted to live with and have sex with someone. (Family planning clinics only started to prescribe the pill to unmarried women in 1974, and that was still controversial.) Even among ordinary families, there was often pressure on young people to 'settle down', without the expectation of 'true love' as an essential.