From a long time nicotine user, I’ll address your bullet points.
Intense: A combination of no tolerance with being sensitive to it.
How do people enjoy: You’re sensitive to it, what you described sounds like a case of nicotine poisoning. When I started vaping and rolled my own coils and all that jazz, I experimented a lot and consumed way too much nicotine in the beginning. The happiness is a lie, as well as the help with anxiety. Nicotine withdrawal feels very similar to being anxious at some stages. It is relief of those withdrawal symptoms that people associate with less anxiety or even happiness.
Are the symptoms related: I could not tell you that.
Addicted: Unlikely you’d be addicted after a single use case, that being said, it feels awful, but the awful tapers after about 3 days - the future cravings mean you’re addicted, in the most tell tale way. I use nicotine pouches as I’ve moved away from smoking. I still want a good old fashioned Marlboro Red, AKA cowboy killers.
Why do people want to keep it up? Often its a situational thing. You smoke with friends on the weekend, but not during the week. Then its daily, because you start feeling withdrawal. Soon you’re smoking a pack a day ten years later kicking yourself for ever trying it.
If you (or anyone else) can avoid nicotine, I advise it wholeheartedly. The simplest way is just not to start, cliched as that may be.