The 1st half of the 10 Commandments are ‘bein adam leMakom’, between man and Hashem, and the 2nd half are ‘bein adam lechavero’, between man and man. The Mabit points out that the ‘bein adam lechavero’ commandments were written on the tablets in much fewer words, though in much bigger font size, than the bein adam leMakom commandments. This was in order to emphasize to us the importance of the mitzvot that are bein adam lechavora. Derech eretz, being a mentsch, behaving correctly towards others, is a pre-requiste to receiving the Torah.
Read more: http://www.shortvort.com/yisro-parasha/11297-3-thoughts-in-one-derech-eretzbeing-a-mentsch-is-a-pre-requisite-for-receiving-the-torah-i-heard-all-3-from-rav-yechiel-emmanuel