What is the connection between the first two posukim?
(19,2) “Speak to all the congregation of Yisrael, and say to them: You shall be holy, for I, Hashem, your G-d, am holy. Every man shall fear his mother and his father, and you shall observe my Sabbaths. I am Hashem, your G-d.”
What is the connection between the first posuk - the command to be holy, and the second posuk - fearing one’s parents?
Behold, it is well known that there are two ways of serving Hashem out of fear - fear of punishment, and fear (awe) of Hashem's lofty greatness. The difference between them is this: It is explained in many places that a person has to sanctify himself even in those things which are permitted to him. For example, not to eat more than necessary, and to conduct himself with his wife as if he was being forced by a demon - not intending for his pleasure but rather only to fulfil the mitzvah of procreation. Because the truth is that even the permissible things that a person does not do out of necessity but just for his own pleasure are very much hated in the eyes of Hashem, because Hashem is holy.
However, the reason why Hashem did not command us in these things, and why if we do not behave thusly we will not be punished, is because Hashem wants his creations to be righteous, and the ways of Torah are pleasant. Hashem saw that if He would decree that one should not intend for pleasure during relations, but only to intend to fulfil the mitzvah of procreation, and concerning eating he would decree not to pursue unnecessary food and drink, then most of Yisrael would not be able to observe these intentions, and so, G-d forbid, most of Yisrael would be liable to go to Gehinnom. Therefore, Hashem decreed that there should be punishment only on those things that everyone can keep. But nevertheless all the things which are permissible, but which are not essential, Hashem loathes that a person should do them.
Now, if a person does mitzvos only because of fear of punishment, then he only observes the negative commandments which Hashem made punishable - he is afraid and so observes them. But in those things which are permissible he does whatever his heart desires, intending only for his pleasure, since he does not fear punishment. But one who has fear of Hashem’s lofty greatness, disregarding hope of reward or fear of punishment, then even in those things which are permitted to him, he sanctifies and purifies himself - because of his love for Hashem, he is afraid to do something which is loathsome in Hashem’s eyes.
Now, Hashem commanded us to fear our father and mother. But how, indeed, is possible to fear one’s parents because of fear of punishment? Because if the son is afraid of his father hitting him because he is not able to strike back, then this fear is not dependent on this being his father, because he would similarly fear anyone powerful who might hit him. And if the son is not afraid of his father hitting him, because he could hit him back, only he will not do so because Hashem commanded him to honour his father, then someone who observes Hashem’s mitzvos only from fear of punishment, even though he fears his father, this fear is not considered fear of his father alone, but rather fear of Hashem’s punishment, and for this reason he honours his father and ‘fears’ him.
But someone who fears Hashem’s loftiness, who recognises that Hashem created him and sustains him, and does for him all good things, and that it is proper to fear Him, if so, he will likewise fear his father and his mother, who gave birth to him, and raised and sustained him, as the author of the Shavuos piyut enumerates. The result is that he is fearing his parents themselves, not because Hashem commanded him. This is the intention of Chazal, when they said that Hashem equated honouring one’s parents with honouring Him. That is, since Hashem wanted that a person should fear his parents themselves out of fear of their greatness, not because Hashem commanded him to fear them, he equated their honour with His honour - just as the main way to serve Hashem is from fear of his greatness, so too He desired that a person should fear his parents because of their greatness.
By this, the mitzvah of honouring one’s parents is distinct from other mitzvos, because by other mitzvos there is no need to fear from the mitzvah, but from the one who commanded the mitzvah. But here it is a mitzvah to fear the parents specifically. This is what our posukim are saying - “you shall be holy”, that is, you shall sanctify yourselves in things which are permitted to you, that is, you shall have fear of Hashem’s greatness. And so through this, “you shall fear your mother and your father” - you shall fear them, themselves, that is, fear of their greatness.
When do the parents deserve to be feared and honoured?
(19,2) “Speak to all the congregation of Yisrael, and say to them: You shall be holy, for I, Hashem, your G-d, am holy. Every man shall fear his mother and his father, and you shall observe my Sabbaths. I am Hashem, your G-d.”
A different, although similar explanation of the flow of the posukim, is according to what Chazal said on the posuk in Tehillim 51:7 “Behold, with iniquity I was formed, and with sin my mother conceived me”, and the Midrash on parshas Tazria teaches that Dovid said: Yishai, my father, intended only for his pleasure. It is clear from there, that in most cases the intention of the parents is not for the offspring, but rather for their own pleasure. But if they do so, then they are called sinners, like it says, “and with sin my mother conceived me”. Rather, the imperative is that one intends only to have offspring. This is also clear from the Midrash on parshas Noach - (Iyov 24:18) “he does not turn by way of the vineyards”, that the intention of the generation of the flood was not to plant vineyards, meaning to have children, but Noach’s intention was only in order to have offspring. In fact, the main sin of that generation was because their intention was not on having children.
Now, the main obligation of honouring one’s parents is because they brought us into the world. But this is only relevant if their intention was indeed to bring us into the world, and this was their main intention. But if their intention was not for this, but rather for their own pleasure, if so, our coming into the world was only from Hashem, and not from our parents, since this was not their intention, and therefore it is not fitting to honour them.
Now, Rashi explained on our posuk “you shall be holy” - you shall separate yourselves, because wherever you find a barrier against sexual immorality, there you find holiness. This is taken from the Midrash and the Yalkut, see there. If so, if one intends in relations only for one’s pleasure, he is not called holy. Rather, one must intend to have offspring for the sake of Heaven - then he is called holy. This is what our posukim are saying: “you shall be holy”, separated from sexual immorality, and intend only for the sake of Heaven to have offspring. Consequently, “every man shall fear his mother and his father”, since they are fitting to be feared and honoured.