When Avraham is negotiating to buy the Ma’arat Hamachpela to bury his wife Sarah, Efron, the owner of the property suggests a price in a very cunning manner. He says: ERETZ ARBA ME’OT SHEKEL KESEF BENI UVENCHA MAH HI, “…a land worth four hundred silver shekel, between me and you, what is it…?” (Gen. 23,15)
Rashi comments on the phrase “between me and you”, between two friends like us what is it worth? What was Rashi’s rational that he deducted from Efron’s words that he implied that they were friends, hence what is four hundred shekel between friends?
A Rabbi once explained it very cleverly. When addressing an important person or a superior, one should say, “between you and me”. It would be a slight to mention yourself before the dignitary. When one speaks to someone they love, it is proper to say “between me and you”. Informality is acceptable.
Hence Rashi explains that he addressed Avraham as someone he loved and it is thus proper to mention himself first.
It is very important when speaking to others that the proper approach and the proper language be used.