"And Hashem made the two great lights"(1:16). The sun and moon were created equal in size, but the moon was reduced because it complained and said, "it is impossible for two kings to use the same crown" (Rashi).
The sun remained the same size because it heard its disgrace by the moon and did not respond. The Talmud (Shabbos 88b) says, "those who are insulted but do not insult, who are disgraced but do not reply, who perform with love and are happy in suffering, regarding them it says, 'those who love Him shall be as the sun going forth in its might" (Da'as Zekainim).
"And it (the sun) is like a groom coming forth from the bridal chamber" (Tehilim 19:6). This pasuk teaches an important lesson to a groom. Notwithstanding mutual love and dedication, a new couple, given the pressures and adjustments in their lives, is likely to experience tension. A new wife may say something that causes her husband to feel insulted. At such a time, the groom is commanded to be like the sun and not reply. By his silence, he will be great like the sun, and enjoy a peaceful marriage.
In fact, men are compared to the sun, a source of light, and women to the moon, who receives and reflects the sun's light. In addition, the sun is the same size and shape each day, whereas the moon's size and shape change each night. This corresponds to gender differences as well. A woman changes more than a man, both physically and emotionally, and this itself is part of her attraction (see Nida 31b). However, a man's role is to always be serene and happy, and a wife relies on this stability.
A husband must talk to his wife calmly, and be neither sad nor angry (Rambam Ishus 15:19). A wife has no such obligation (ibid 20), as by her nature she is more sensitive, and as such, more given to tears. The husband must be supportive, and, like the sun, be a constant source of strength, never replying to an insult.
The Chafetz Chaim (Shmiras Halashon, Sha'ar Hat'vuna 8) points out three ascending levels described in the Gemara (Shabbos 88b, quoted above): 1. A man does not insult someone who insulted him, but does respond 2. A man does not respond at all to an insult, but is bitter in his heart 3. A man does not respond, out of love for Hashem, and accepts the pain of insult with joy. One is required to be happier over suffering than over good (Tanchuma), and this includes suffering an insult.
In this vein, the second part of the description of the sun, rejoicing like a warrior to run the course, is apt for a groom as well. By not responding, and even rejoicing, when insulted, the husband reaches true greatness.
The warrior is happy because he is confident in his strength (Metzudos). At first glance, a confident person is more likely to respond to, and even insult, one who insults him. But in reality the opposite is true. One who lacks self-esteem is more prone to respond and insult in order to defend his honor. A confident person, like the sun, need not engage in verbal self-defense.
Indeed this lesson applies to a husband throughout his marriage, in all situations. In this way he can run the course of a long and happy married life, and be like the sun going forth in its might.