1.) Their names used to frighten the enemies on battlefields.
After Guan's capture of Yu, SGZ says "羽 威 震 華 夏," (Guan affrighted the whole North China Plain.)
In comparison, when "先 主 遣 人 迎 超 ， 超 將 兵 徑 到 城 下 。 城 中 震 怖 ， 璋即 稽 首 " (Liu Bei accepted the submission of Chao, Chao led a troop to ChengDu. After hearing of Chao's arrival, ChengDu was horrified; Thus Liu Zhang immediately surrendered.)
2.) They were both excellent on combat.
I think I need not to give examples about no.2 statement.
From above 2 points, we can see they deserved a grand reputation of military, and both of them were fierce fighters.
However, these were also their origin of conflict.
According to SGZ:
羽 聞 馬 超 來 降 ， 舊 非故 人 ， 羽 書 與 諸 葛 亮 ， 問 超 人 才 可 誰 比 類 。 亮 知 羽 護 前， 乃 答 之 曰 ： 「 孟 起 兼 資 文 武 ， 雄 烈 過 人 ， 一 世 之 傑 ，黥 、 彭 之 徒 ， 當 與 益 德 並 驅 爭 先 ， 猶 未 及 髯 之 絕 倫 逸 群也 。 」 羽 美 鬚 髯 ， 故 亮 謂 之 髯 。 羽 省 書 大 悅 ， 以 示 賓 客。
It briefly says when Yu heard the news that Chao submitted, he sent a letter to Zhuge, asking who Chao could compare. Zhuge replied: "Chao's talent is all- round, especially he is very vigorous. Chao is like the ancient heros in modern days, he can compete with Zhang Fei; however, you are the only one who is unsurpassable." Guan was thrilled to have Zhuge's reply letter, and he showed the letter to all his guests and visitors.
From the information gathered, I find another character of Guan, except for his arrogance, he was jealous.