I agree with all the reasons stated above. I just want to add another reason that I haven’t seen anyone here mention yet.
Sometimes the help is not really an act of altruism, meaning that the help is less about the help receiver but more about the helper. In other words, the helper wants to help to validate themselves.
Some people are just turned on by seeing that they successfully help others. Granted, most of us feel good when helping people one way or another, but some people just take it too far. They want to feel important. They want the recognition from others that they help some particular people, the recognition that they are generous, kind, whatever. And if the person they try to help change in the same way as their vision, they feel an even stronger sense of entitlement because it proves that they are right after all. They are even more turned on.
I’m not against giving help, quite the opposite. I just want to say that when you give help it shouldn’t be for a completely selfish reason.
Source: I’ve been there before, being drained by a narcissist trying to “help” just to prove to everyone that she was kind and important.