I am not sure if Harris' order is significant. Anyway, this is the one he highlights first.
Dating tends to skip the friendship stage of a relationship
CS Lewis describes friendship as two people walking side by side towards a common goal. Harris suggests that dating, due to its focus on physical attraction, on the fact that you are now (in some sense) 'an item' or 'a couple', can so easily lead to superficiality and to a stifling of the growth of the friendship. There is not that freeing aspect of being able to be yourself and do things together because you enjoy them because you are focusing too much on looks, the 'feelings' you have for each other etc. (I Kissed Dating Goodbye, p38-39)