All the evidences I saw indicated that Diana wanted a marriage with Dodi, and it was Diana who proposed the idea of marriage on Aug 28 or Aug 29, which drove them to detour to Paris, where Dodi got to buy the ring "Tell Me Yes".
On Aug 29, Dodi'd already told friends they were going to marry, at that time he even hadn't prepared an engagement ring. The logic here is, if it was Dodi who proposed the idea of marriage, he would have been much more well prepared than he was -- at least he would have prepared a ring first. What gave him the confidence to be so assertive about this marriage when he didn't even have an engagement ring ready? I think such assertion is because it was Diana who proposed the marriage, not Dodi. That's why he didn't have an engagement ring prepared.
On the other hand, Diana did make a lot of preparations for the marriage. As early as Aug 13, she gave Dodi her beloved father's cufflinks to Dodi (Diana called her father the man she loved most in her life, who left Diana 5 million pounds in his will, far far larger than what her two sisters got). At around the same time, she went to consult a priest, Father Frank Gelli, whether he could perform the marriage service for them.
Father Frank Gelli spoke with an excited Diana shortly before the fateful crash in a Paris underpass on August 31, 1997. At the time he was curate of St Mary Abbots church in Kensington, west London, near her former home at Kensington Palace. Princess Diana started going to the church a few months before she died, slipping in at the back during services and trying to keep a low profile. Recalling one conversation, Fr Gelli told how she stopped to speak to him on the way to the gym. He said: “She wanted to know if it was possible for two people of different religions to marry. I told her it was.
“As we spoke her telephone rang. It was obviously Dodi. Her eyes lit up. As she was leaving she asked me if I would be able to perform the service when she got married. Her love was obvious.”
While on holiday cruising the Mediterranean on Mohamed Fayed’s yacht, the Jonikal, she called Fr Gelli and said: “I have good news.”
She asked him to meet her at Kensington Palace when she returned, but days later in Paris she died along with Dodi, 41, in the crash.
https://www.express.co.uk/news/royal/837400/princess-diana-death-princess-of-wales-dodi-fayed
Some people doesn't believe the "Tell me Yes" ring was an engagement ring, because it was too cheap, merely a little more than 10,000 dollars. But that was not all the gifts. There were two charities which very dear to Diana's heart coming with it. She told Richard Kay on phone six hours before her death, Al Fayed had agreed to finance a charity for landmine victims, and she and Dodi would set up hospices around the world.
Why would Al Fayed have promised to finance a charity for landmine victims, if Diana was going to dump his son soon?
I think in Al Fayed's mind for a long time, he believed it was a coming baby which lead to such a quick decision of marriage. Well, in 99% of all cases, a quick marriage is due to a coming baby. But I think Diana's case is within that 1% group. I think it was because at that stage of her life, she urgently needed someone who can protect her and make her feel safe. And recent events had also indicated that Al Fayed's had adopted such a belief. In an interview on the 20th anniversary of his son and Diana's death, he said,
"Princess Diana trusted my son and he became a shoulder to lean on at a troubled time in her life. Dodi wanted to be a safe haven for her and her boys. Our hearts go out to her sons. They lost a devoted mother and a wonderful human being. The anniversary is a particularly difficult time for my family and I. The reality is we suffer grief every day of our lives."
https://www.thefreelibrary.com/Dodi+'just+wanted+to+keep+Princess+safe'.-a0501879617
Mr. Fayed's words echo Diana's words to her friend Rosa Monckton two weeks before her death about the luxury gifts Dodi gave her, "That's not what I want, Rosa,it makes me uneasy. I don't want to be bought...I just want someone to be there for me, to make me feel safe and secure." [1]
There are a lots of other evidences suggesting such unsafe feeling rooted in her mind at that stage, but let's not spread it out here.
Reference:
[1] “Friends Say Diana Adored Fayed, Forgiving Toward Charles, Others”, Chicago Tribute, 8 Sep 1997.