The Rambam reads the command to obey Beis Din,“lo tasur,” found in parshas Shoftim, as the source of our obligation to obey all dinin derabbanan. Ramban disagrees. He argues that if our obligation to follow dinin derabbanan stems from a pasuk, then every violation of a din derabbanan is potentially a violation of this d’oraysa of lo tasur. There should therefore be no difference between dinim derabbanan and dinim d’oraysa – but there certainly are differences, e.g. sfeika d’oraysa l’chumra, sfeika derabbanan l’kula.
According to the Ramban, if “lo tasur” does not bind us to follow dinim d’rabbanan, what does? In the past (here, here) we discussed R’ Elchanan’s answer to this question: the Chachamim reveal to us what the ratzon Hashem is; Hashem’s will obligates our obedience even without a formal command. R’ Shimon Shkop offers a slightly different answer that I can only describe as very R’ Shimon-ish. Here’s a link to the page in Sha’arei Yosher (I:7 d”h v’nireh). R’ Shimon suggests that logic (the seichel) and logic alone is sufficient to obligate one to follow Chazal. Chazal’s enactments are by definition just and good. One does not need a new mechayeiv, a new legal mechanism, to create a sense of obligation to do that which is right – the sense of obligation to do that which is just is innately part of our being.