Why were Gimli, Legolas, and Boromir chosen?

I believe that after Frodo had offered to take the ring, Aragorn chose to defend Frodo because he knew how important it was that the ring didn't get into the hands of Sauron again. Legolas seemed eager to join along side Aragorn because he was to be king, and so was Legolas, thus making a good bond between the Kingdoms of man and elf. Gimli joined because he didn't want the fate of the ring in the hands of an elf. Boromir went along because he wanted the ring for use in battle againist the enemy. Boromir probably planned to steal the ring from Frodo, which he attempted at the Falls of Rauros. Peregrin(Pippin) and Meriadoc(Merry) joined along because they refused not to go without Frodo.