Berakhot 17

The gemara lists prayers that rabbis would add at the end of their shmone esreh or at particular moments. What is the root of the debate between the rabbis and Rabban Shimon ben Gamliel regarding whether or not a groom can say shema on his wedding night? Is the issue that he not appear arrogant? If so, this seems the exact opposite of their debate regarding working on Tisha B'av. One is exempt from mitzvot on the day a close relative dies before the burial. Also those carrying the body are exempt from certain mitzvot - depending on how needed they are.