why did the writer make David attract to Steer forth and get as a modal even though he represents the bad side? What does this symbolise?

steer forth and Traddles are on the contradectory to each one character, one represent the evils of high class corruption the other stands for the lower class.

but the writer made David to attract to the bad side, which in here is Steerforth. What does symbolize?

2- eventhough many events showed to David that this character isn't good he kept denying. till the last one when he hurt a dear person to him