Is this an invention of Maimonides?
Here is the source for not imposing excessive fear on her:
Gittin 6:
Rav Hisda said: A person should never impose excessive fear within his household, for the concubine at Gibeah—her husband imposed excessive fear on her, and this caused many tens of thousands in Israel to fall.
Rav Yehuda said in the name of Rav: Anyone who imposes excessive fear within his household will ultimately come to three transgressions: sexual immorality, bloodshed, and desecration of the Sabbath.

You asked specifically about sadness. To that I answered that no source is needed. Straightforward common sense is enough. And I added that imposing fear too, which certainly does have a source in the Talmud, is also a result of common sense. They learned it from the concubine at Gibeah (yeah, right. Obviously that’s not really a source; they just latched onto it) based on a common-sense consideration. That’s all.

So Maimonides added the matter of sadness on the basis of common sense, or is there a source for it in the Talmud—that’s what I wanted to know. You’re claiming that sadness has no source, and Maimonides wrote it on the basis of a consideration of common sense.
Thank you very much!

Maybe “and gladden his wife”?

Besides that, see Yad Peshutah on the Laws of the Foundations of the Torah:
“‘And not sadness’—compare Guide of the Perplexed I:29: ‘Etzev is a homonymous term. It is a term for pain and sorrow—“in pain you shall bear children” (Genesis 3:16). And it is a term for anger—“and his father had never saddened him all his days” (I Kings 1:6), meaning: had never angered him … And it is a term for rebellion and defiance—“they rebelled and saddened His holy spirit” (Isaiah 63:10), “they saddened Him in the wasteland” (Psalms 78:40) … And in the sense of the second or third meaning it is said, “and He was saddened to His heart” (Genesis 6:6). If according to the second meaning, its explanation is that God became angry with them because of their evil deeds … But the explanation of “and He was saddened to His heart” according to the third meaning would be: man rebelled against God’s will concerning him, for heart also refers to will.’”

With God’s help, 8 Tishrei 5783

According to Maimonides in the Laws of Character Traits, the balanced emotional state is “not frivolously joyous and wild, nor sad and mournful, but cheerful with a pleasant countenance,” so avoiding sadness is also for the person’s own good.

When he is married, he is also obligated to build an emotionally supportive world for his wife, as part of the marriage contract commitment: “I will honor… and support you.” In order for the husband to honor his wife and carry her emotional burden, he must be in a balanced state of “not sad and not irritable,” but rather “cheerful with a pleasant countenance.” That is how the wife receives a sense of security and emotional support.

With blessings,
Shemaryahu HaLevi Oxytocinski