According to the Torah (see Sefer Vayikra 6:20), when something comes into contact with a sacrifice and absorbs some of its taste, it takes on the laws of that sacrifice. So if a loaf of sacrificial bread came into contact with a sacrifice, it becomes limited by the same parameters of the sacrifice, i.e. if the sacrifice becomes disqualified the loaf cannot be eaten; even if the sacrifice is valid it can only be eaten at the time and in the place where the sacrifice can be eaten.
The Gemara presents a simple question. Given that the prohibition to eat the loaf under these circumstances is a mitzvat lo ta’aseh – a negative commandment – and eating the sacrificial loaf is the fulfillment of a mitzvat aseh – a positive commandment – shouldn’t we apply the principle aseh doheh lo ta’aseh – that performing a positive commandment “pushes aside” the prohibition – and the loaf should be eaten!
Many have asked what the underlying reason is for the rule that aseh doheh lo ta’aseh, given the general sense that we have that biblical prohibitions are more severe than positive commandments, since there are punishments for transgressions, but no punishments for merely neglecting to perform a mitzvah. Rabbenu Nissim Ga’on suggests that this rule is built into prohibitions, that they do not apply when a positive commandment stands in its way – an explanation that appears in the Talmud Yerushalmi, as well. In his Commentary to the Torah (Shemot 20:8) the Ramban argues that the performance of positive commandments is based on the love of God, while negative commandments are based on the fear of God, and that love is greater than fear.
Neither the Gemara nor the rishonim explain why this rule does not apply in the Temple. It appears that the service in the Temple, which is restrictive in many ways and does not encourage individual creativity in worship, limits a rule such as this one.