Berachos, Chapter 2, Mishnah 8. This mishnah relates back to what was taught above in Mishnah 5, where the incident was brought of Rabban Gamliel who recited Shema on the first night after he married a woman, even though by the letter of the law a groom is exempt from reciting it on that night.
The words of the Mishnah:
"Chasan, im ratzah likros kerias Shema laylah harishon, korei" - a groom, even though by the letter of the law he is exempt, is permitted to recite it, just as Rabban Gamliel did.
"Rabban Shimon ben Gamliel omer: lo kol harotzeh litol es haShem yitol" - in his view, not every person is entitled to conduct himself this way.
What is the groom who recites expressing?
Conduct of piety.
Precision in mitzvos.
The ability to concentrate even while he is preoccupied with another mitzvah.
Taking a stricter standard upon himself, beyond the law.
Yet there is a certain measure of pride in such conduct, and his act appears as arrogance, and therefore it is not fitting for a person to behave this way.
The words of Tosafos as they apply nowadays:
Tosafos write that nowadays we cannot claim that we are accustomed to concentrating with precise intent while reciting Shema. Therefore, a groom who refrains from reciting Shema on his wedding night - his very refraining is itself a form of arrogance, as though he were declaring that his concentration is complete on a regular and constant basis, to the point that he is unable to concentrate properly now. For this reason, a person nowadays is permitted to recite Shema on the first night of his marriage.
In summary: the mishnah teaches that a groom who wishes to recite Shema on the first night may recite it, like Rabban Gamliel; and Rabban Shimon ben Gamliel disagrees and rules that "not everyone who wishes to take the Name may take it," because of the measure of arrogance involved. Tosafos teach a novel point, that nowadays the ruling is reversed: since we are not accustomed to precise concentration, it is specifically refraining from the recitation that constitutes arrogance, and therefore the groom is permitted to recite.