Okabe’s feelings of guilt towards Mayuri
According to the Steins;Gate Ouroboros light novel, Okabe started distancing himself from Mayuri around the time he started middle school. He did so because he was embarrassed to still be playing with his girl childhood friend. After that, Mayuri began spending more time with her grandmother. Less than half a year after that, Mayuri’s grandmother passed away, and for the next six months, Mayuri didn’t speak a word. Until the day Okabe made Mayuri his hostage.
The point of all this is that when Mayuri stopped speaking, and when Okabe thought she was going to disappear, he thought it might be his fault. So guilt was one of the driving forces behind his becoming ‘Hououin Kyouma’ to save her.
Now, there are various differences
in the light novel compared to the VN, so this doesn’t necessarily
all apply to the VN. But I think it would make a lot of sense. It
also strengthens the parallel between this event, and Okabe trying to
prevent Mayuri’s death, which he also saw as his fault. (‘If only I
hadn’t told Daru to hack SERN,’ 'If only I hadn’t insisted that we make a
time machine,’ 'If only I hadn’t made Moeka into a lab member.’)
Of course, Okabe would have tried to save Mayuri even if he didn’t feel like it was his fault. But the guilt just makes it that much worse.