As we know, Tywin Lannister's attack on King's Landing resulted in death of Prince Doran's sister Elia and her children with Prince Rhaegar. The Martells never forgot that it was the Lannister bannermen who did it and Prince Oberyn suspected it was Tywin's payback for Aerys choosing Elia to be Rhaegar's bride instead of Cersei.
When the War of the Five Kings began, it is curious that Doran did not declare for any side. It makes sense why he would not join the following:
- Balon Greyjoy - Too distant, small chances of success.
- Robb Stark - Too distant, small chances of success.
- Stannis Baratheon - Little to no chance of success.
- Joffrey Baratheon - Cersei's son, Grandson to Tywin, supported by might of the Rock.
That leaves one man, Renly Baratheon. Renly was right next to Dorne and had a huge army, was beloved by the commons and had the wealth of Tyrells behind him. He had the best chance of victory. If Doran were to support him, he could finally get revenge for Elia and get the new King to destroy House Lannister. Yet, he did not.
If avenging Elia was all that he wanted, why not join Renly?