Positive Commandment # 37 (Aveilus)

Maimonades in Sefer Hamitzvoth says that A Cohen must become Tameh to his six close relatives. The source for this obligation is the Passuk "Lah Yitamuh". Maimonades continues to state that this is the source for the general obligation that everone must mourn their six close relatives (mideoraysah one day). However, only a Cohen must become Tameh while a Yisroel just has to mourn. This Rambam seems strange indeed, especially in light of the fact that the Rambam in Hilchos Aveilus 1.1 brings a different source for the obligation to mourn. That source is the statement of Aharon Hacohen "Ocholti Chatas Hayom Hayitov Be'enei Hashem". The Keseph Mishna in fact asks why the Rambam brought a different source. He continues to ask that seemingly becoming Tomeh has nothing to do with availus. Proof in point is the fact that a Cohen has an obligation to become tomeh to a meis mitzvah and obviousely has no din aveilus regarding the same. Further complicating matters is the rambam in Hilchos Aveilus 2.6 "kamah chamurah mitzvas aveilus" The rambam states that it is so strict that even a cohen who otherwise is not allowed to become tomeh becomes tomeh to a relative. We see from this rambam that in fact "lah Yitamuh" is a fundemental part of the mitzvah??????? (as an aside, the language the rambam uses "kamah chamurah" seems out of place in that how do we see a chumrah when the mitzvah is to become tameh?!!

Strange indeed. One Mitvah with two different sources, telling us two seperate halachos which do not logically have much to do with eachother.