Devo Bhava (देवो भव)

Following mantras are quite common.

मातृदेवो भव |  पितृदेवो भव | आचार्यदेवो भव | अतिथिदेवो  भव |

Taking last shloka(अतिथिदेवो  भव) as an example, I always thought that it meant Guest is god. But recently i learnt a different meaning of the same.

These sholkas are told by Gurus in gurukool to their students. The sholkas appear in प्रथमा शीक्षावल्ली of तैत्तिरीयोपनिषत. The sholkas are in imperative mood - its teacher telling students अतिथिदेवो  भव. In this context the correct meaning arises which is  - teacher telling students "You become a person who treats guest like god" . Same applies to other sholkas.