Canon Lawyer Dr. Ed Peters of Sacred Heart Seminary has a post on Canons 915 and 916 in light of the current questions of interpretation on how to conduct the giving or receiving of Communion by those who are notorious grave sinners or otherwise not in communion with the Church.  Dr. Peters hits the main difficulty from the human point of view:

To be sure, both canons make serious demands on the faithful.
It’s not easy for an individual Catholic to refrain from going to holy Communion at Mass. The so-called ‘Communion fast’ offers no cover for a Catholic with a doubtful, let alone a guilty, conscience. These days, to remain in the pew while everyone else goes to Communion is tantamount to saying “I think I’m in the state of grave sin.” Who wants to imply that?

We are weak human beings, and it takes a lot of strength to do such a thing.  Of course, strength, robur (which literally implies the steadfastness of an oak tree) is the effect of one of the other sacraments, namely, Confirmation, the sacrament that makes you “firm”.  In this line of thought, St. Thomas has this interesting statement, which, as far as I know, he only says once in all of his works:

…for all the other sacraments are seen to be ordered to this sacrament [i.e., the Eucharist] as to an end.  For it is manifest that the sacrament of Orders is ordered to the consecration of the Eucharist.  But the sacrament of Baptism is ordered to the reception of the Eucharist.  In this, someone is also perfected through Confirmation, so that he may not be afraid to withdraw himself from such a sacrament…

An interesting thought: an effect of Confirmation is the fortitude not to go to Communion when you shouldn’t.  This could certainly be useful today, when many parishes have the enforced courtesy squad in the form of ushers who let us know when it is our turn to receive Communion.

Of course, in St. Thomas’ day, the reception of Confirmation generally preceded the reception of first Communion in the West.  Today, it is not so, at least in the U.S.  And I don’t think that the practice has to change just because of this consideration.  Confirmation itself does not carry in its notion any determination as to age; its determination is left free to the discretion of the Church’s ministers.   Nevertheless, I think it would be nice if by some determination of positive law, we made it easier for conscientious Catholics to “withdraw” from the reception of Communion when they judge they should not.  For that, I support Dr. Ed Peters’ own proposals for modifying the Communion fast.