Mr. Darcy's first and foremost motivation at the beginning of the novel Pride and Prejudice was precisely that: His pride. The need to preserve a very stern sense of dignity among people whom he, due to his upbringing and social interaction, considered to be lesser than himself.
This did not mean, as we will see, that he was a bad person. Eventually all that was pride began to be substituted with understanding. Most of this understanding was thanks to Elizabeth, whose common sense drilled a huge hole into Darcy's ego and let him realize that there is much more to people than their social rank and status. His motivation then changed to the love he felt for Elizabeth and the interest that he had in clearing her family name (stained by Lydia) in order to make her more acceptable to marry him.