I believe Sesshomaru had a sheltered upbringing. He was likely not permitted to have friends, because other demons were “beneath him” (also literally bc castle in the sky lol) I’m sure he did not have many, or any friends growing up, hence his terrible social skills.
I believe Sesshomaru wasn’t allowed to leave the castle, so he probably didn’t see much of the world below the clouds. That’s why he’s always gazing at the moon, and staring off. Or doing this (????)
I don’t believe that his hatred for humans was taught. I believe it was developed when his father died. He was probably taught that all beings were beneath him, but his hatred and resentment were emotional.
Sesshomaru is generally quiet, so I’m sure he was taught not to speak unless spoken to. He was probably neglected and casted off a lot, told to find ways to entertain himself. His father was probably too busy with his role to spend much time with him.
There isn’t any background about how or when his father left his mother, to marry Izayoi but maybe that caused him some abandonment issues? He only trusted people that would serve him, because this meant they would not abandon him.
Sesshomaru was very adamant about Rin fending for herself and having freedom, even if he knew she would need help (which is why would send Jaken to look after her). He wanted her to have the independence that he likely did not have because he wanted her to be happy.
Sesshomaru probably always resented his lifestyle. And after losing interest in supreme power, I’m sure giving up his title to live a more peaceful life was what he always wanted.
I can tell by his attitude that everything was handed to him, and he was raised to believe he could have whatever he wanted, even if it was someone else’s.
I think he tried to impress his father constantly. I’m sure surpassing his father and being the rightful heir of Tenseiga was important to him in the past. He didn’t understand why his father would put him through such things, if he felt he did no wrong by him.
Age commonly progresses more slowly in stories of supernatural beings. Some are still children/young adults at hundreds of years old. (Shippo is 50, apparently) So Sesshomaru really may be 19 years old physically/mentally. And even younger emotionally. He did have a lot to learn.
His mother may have taught him that certain emotions are “annoying” or “unbecoming” so he doesn’t express himself very much. Which is why his own mother couldn’t even connect with his feelings in the final act, and Jaken had to relay his true feelings to her.