Orthodontic treatment can be carried out without extractions, but this depends on each individual case. Arch expansion is usually carried out when the arch is too narrow, however this may not be the case with your DD.
Your orthodontist proberbly gave you advice, which would create the best asthetic outcome, however, if you are happy to accept a slightly less asthetically pleasing result and have no extractions, then you should discuss the treatment fully with your orthodontist.
Sorry, can't really comment more, as each case is very different.