http://revach.net/article.php?id=4708

Section: Parshas HaShavua   Category: Quick Vort
Parshas Shoftim: Rav Moshe Feinstein - The Only Matzeiva You Can Make Is On Your Grave
"V'Lo Sakim Licha Matzeiva Asher Sanei Hashem Elokecha; Do not erect a Matzeiva which Hashem hates." (Shoftim 16:22).  Rashi explains that although Hashem loved the Matzeivos in the times of the Avos, now Hashem hates them.  Why?

Rav Moshe Feinstein says that we find Yaakov Avinu made two Matzeivos.  On one he poured Nesachim, and one was for Rochel when she died.  Although the prior one is forbidden today, we still make matzeivos when people die.  Rav Moshe explains that the inyan of a Matzeiva is that we erect before Hashem the mitzvos we have done, and ask Hashem to recognize the reward we deserve.  

Before Matan Torah no one was commanded to do the Mitzvos, so if someone did a mitzva he could demand his reward from Hashem even if he had no plans of doing mitzvos in the future.  However, now we are obligated to do mitzvos, so we cannot demand our reward until after we die since the obligation continues until then. 

Yaakov made a matzeiva for Hashem by doing mitzvos and in return he asked Hashem to accept his Nesachim.  This we cannot do today because we cannot demand our reward halfway through our mission.  However, we can make matzeivos for people who finished their mission on this world and can now receive their eternal reward, just like Yaakov did for Rochel.