The book I read - which was very measured I thought - about Diana describes her in the time leading up to her death enjoying her relationship with Dodi but not planning for it to become permanent. She said the same thing to several friends about marriage/engagement and pregnancy: "I need that like a rash on my face" - one of her favourite sayings.
She knew that most, if not all, of her power and/or appeal lay in the role of the spurned, heartbroken, rejected Virgin bride of Charles, bravely carrying on for the sake of her children - the Virgin Mary-like mother of the future king. (Despite her affairs, that is how a lot of people saw her, especially after her death, see this thread for proof.) A lot of interest in her would abruptly abate should she marry and live happily ever after with a new man. If she had another child with him the spell would be well and truly broken. Therefore she would have to be sure it was 100% what she wanted before she would consider it. She was fond of Dodi and having fun but they had only just met, she wasn't going to rush into anything. Look what happened last time she married a bloke she had had 11 dates with!
I also remember reading that she had realised that Dodi was going to buy her a ring but had decided (possibly after talking with Paul Burrell so maybe it isn't true) to treat it as a friendship ring and not an engagement ring. Dodi, perhaps was hoping for the latter, and it would only be human nature to confide those hopes - to his father. It may well be true that Dodi was planning to propose but it doesn't mean for a minute that she would have accepted. But Mr Al Fayed, with a father's pride, probably believes that "of course she would have married my lovely son," so his claims they were getting engaged may well be true to him.