He explains why it is a custom to not eat at each other’s home for reasons of Kashrut. And why it is not so all year long. Also, even if there is one of whom it is known that there it is completely Kosher, even better than in one’s own home, still the custom is to not eat. This is so because the prohibition on Hametz (leavened bread) is on anything, and it is impossible to guard oneself from anything. Rather, the Creator can watch over for him, that he will not transgress even with anything.
This is why it is written that with leavened bread, you should be careful with anything. One is commanded to caution, and he should seek advice how not to come to “anything” leavened.
However, one cannot guard oneself. Hence, only the Creator guards. And certainly, the guard is in such a way that not everyone is equal. Some are better guarded by the Creator, and some are less guarded, depending on one’s need. This is so because there are people who know that they need great care, so they draw greater care, and there are people who feel that they do not need such guarding from Above. Also, this cannot be said, as it depends on the sensation: some feel themselves deficient, and need greater care.