Imagine you have a friend who's an alcoholic. You know, they're kind of a good person, fun to hang out with once in a while, some good qualities, but they keep showing up drunk and getting these crazy ideas that don't make any sense, and they have a bunch of other annoying bad habits and tendencies. It's not enough to make you stop hanging out with them, but it really gets on your nerves to the point where you're kind of relieved when they leave you alone. This friend is Toaru Majutsu no Index.
Then imagine that this friend goes to rehab and comes back a wholly improved person. I'm talking, like, they really cut back on their bad habits, they're more humble and respectful in everything they do, and all of a sudden you really like being around this person a lot more. It's still the same person, and it's not like they're flawless now, but it feels like you took the qualities that made them a likable person before, and filtered out all the crap that really pissed you off about them. Suddenly you're not ashamed to call this person one of your close friends. That's Toaru Kagaku no Railgun.
Now imagine that this same friend starts getting cocky and starts relapsing. They show up on your doorstep drunk again, shouting incoherently, maybe not as much as before, but it's still annoying. Sometimes they have these little moments that remind you what a good person they were right after rehab, but you can see that it just didn't last. Can you imagine how frustrating and ultimately depressing that scenario is? You thought they were cured, but in reality it was only a temporary lapse -- isn't that just totally irritating? That's how I feel about Toaru Majutsu no Index II.