This article contains spoilers for Legion of X #3.
Marvel has finally revealed the real reason Juggernaut hates his step-brother, the X-Men's Professor X. The sibling rivalry between Cain Marko and Charles Xavier is the stuff of legend. One is numbered among Earth's most powerful telepaths, while the other has claimed the unstoppable power of Cyttorak, a God of Destruction, and become his avatar on Earth.
The relationship between Juggernaut and Professor X has always been a strained one, tied to some sort of secret mutant war Professor X's parents participated in when they worked at a facility at Alamogordo, New Mexico. It only worsened after Cain Marko became the Juggernaut, because he immediately dedicated himself to destroying his step-brother. He became one of the X-Men's first, and most dangerous, enemies - launching attack after attack on Xavier's School for Gifted Youngsters.
Legion of X #3, by Si Spurrier and Jan Bazaldua, subtly reveals the real reason Juggernaut hates Professor X. Like so many bullies, Juggernaut is afraid of feeling weak; although his body is weak, he's well aware of the vulnerability of his mind. Presumably this is why Cyttorak granted Juggernaut a helmet to nullify psychic attack while he is wearing it - because he needed the reassurance he would be protected from Professor X's powers. This deep-rooted fear would perfectly explain why Juggernaut has never been able to resist returning to challenge his step-brother time and again.
Professor X has given Juggernaut good reason to fear him. So many battles between Juggernaut and the X-Men have culminated in the mutants knocking his helmet off, leaving Juggernaut vulnerable. When Professor X became Onslaught, he chose to target Juggernaut first of all, ravaging his mind and proving himself superior once and for all. Juggernaut was left in a coma, and upon awakening he was left running madly in panic.
Si Spurrier is one of Marvel's best writers, with a deep understanding of character-work that allows him to shine a light on the inner world of classic heroes and villains like Juggernaut and Professor X. In the case of Legion of X #3, he's finally revealed the real reason for a rivalry that has been part of Marvel Comics lore since the 1960s. The explanation is perfectly in keeping with the history of both Professor X and Juggernaut. Ironically, it suggests Juggernaut and Professor X are more alike than they would like to admit, for both revel in their power. There have been occasions where Professor X too was unable to resist seeking Juggernaut out to face his fears. Every clash just made the bitterness of their rivalry worse, and it is only now - in the X-Men's Krakoan Age - that they are learning to move beyond this.