Nicotine wears off very quickly. While it can remain detectable in your system for days, the feeling it gives only persists for 10-15 minutes after inhale, if that. Varying the dosage only really affects the intensity, not duration.

Speaking of intensity; if you don't have a tolerance, high levels of nic can be physically and mentally uncomfortable. Someone who can comfortably inhale 3mg/ml of nic might get slightly nauseous and uncomfortable even at 6mg/ml, definitely at 9mg/ml, unless you have a counteracting agent in your system like alcohol (which is why "leveling out" is a thing).

The chemically addictive properties nicotine has is only one part of the story. There's also the "throat hit"; the feeling of inhaling something warm and slightly uncomfortable. That's a major part of the addiction.

It also tastes great. And its "something to do"; kind of like a fidget spinner, it keeps you busy.