Shigure Tendo was unskilled at thinking.
He wasn't as smart as others in the same age group. To be rather blunt, his intelligence was somewhat low. Ever since he was young, he was considered a bit of an idiot.
It would have been fine if he had only been average at Spiritual Abilities despite being the direct descendant of one of the seven major Yin Yang Master families, the Tendo family. The bad part was that the most basic spells that other children of the same age could learn so quickly would always take him more than ten times the amount of time to barely manage to learn.
It was obvious that such a child like Shigure Tendo wouldn't be well thought of.
What made things even worse was the fact that Shigure had a genius older brother—Raizen Tendo.
Raizen was amazingly talented, to the point where high expectations were placed upon Shigure from birth. Yet Shigure turned out to be an idiot, making everyone greatly disappointed.