Two of my aunts (my mum's sisters) are, I very much believe, millionaires, and both for different reasons though they both adopt a similar technique to keep their cash flow coming.
Aunt 1- Married my uncle at 19, he worked for the military, she studied advertising (I believe), they both evolved in their career, she earned a good salary for an ad company, he ended up working as an engineer in submarines for the marines, she started her own business, he moved to the petrol industry (meaning he makes loads of money but isn't around to spend it!) and they started investing in properties around the country to rent out. They never really struggled for money and just put a lot into their career (my aunt has never been a stay-at-home mum so never had a break in career even though she could have afforded to) as such they have always had money and always have been quite the spenders, they definitely aren't the most generous people around though.
Aunt 2- Worked in communication on an average salary, met my uncle who worked as an employee for a landscape company (he had grew up dirt poor), they got together and he decided to create his own landscape company and my aunt quit her job to join in on the business. The business has done quite well and they started investing their money in other ventures (properties to rent out, their own car wash company, etc...) and they are now loaded, you wouldn't know if you see them though, they do own a sports car worth the price of an average house (though she seems to live in the garage) but they used an old banger FOR YEARS as a car to go around as they didn't see the need for a newer car if that one still worked, they had a baby two years ago and my aunt got a lot of things second-hand instead of new, she will stand at shop questionning and considering whether or not she can afford a £10 top when it's clear she could probably buy the store if she so wanted to, but I think them growing up in either poverty (for my uncle) or middle/working class household (for my aunt) shaped their attitude towards money. They had to fight to get where they are at and therefore I believe are conscious it could all go away (though it's very unlikely).
The similarity between both of my aunts is that they were clever enough to invest in more than one business and at the stage they are at, they own enough properties and businesses that if one fail, it will barely have an impact on their quality of life, if at all.
I am not sure being a millionaire matters though, I think reaching a level where you don't actually have to worry about money or bills and the odd treat is good enough. Aunt 2 has loads of money but she did really struggle with fertility and I am wondering whether or not she is actually happy (her relationship looks a bit crap from the outside), I think it's nice they don't have to worry about money, and I have learned a few things from their success story that I plan on implementing myself, but I don't necessarily aim to be a millionaire. I can't see how someone who is a millionaire is happier than someone who is quite comfortable (having lots of money can also bring its own issues).