Why does everyone in this thread like posting definitive statements like this? As if it's universally true?
Life isn't as black and white as people like to think.
I was not much of a happy person before I found my (now) wife many years ago. She helps me deal with life in a way no therapist could ever do (and vise versa, me to her). We're in it together and help each other.
I could not figure out how to be happy on my own. Once I found my partner, I was able to be happy.
This isn't universal, of course. Many people are able to be happy on their own, but it's so strange reading definitive stuff like "there's no way!" when it's just not black and white.
See, for example:
https://academic.oup.com/ije/article/36/6/1244/819019
A death of a parent is identified as one of the most significant causes of unhappiness. If your happiness is negatively impacted by the death of your parents, is the issue that your parents died, or that you've given away your agency to choose your own happiness?
You have incomplete and varying, but nonzero levels of control over all these things.
I agree with the whole "if you're not happy alone" rule of thumb but happiness isn't a binary. It's a scale. And amassing a little more wealth, becoming a little more respected, having a partner, all those things can add amounts of happiness that push someone from "unhappy" to "good enough". Look at the reverse case when people lose all that stuff if you really want to see how evident it is.
It sounds good but it is BS e.g., money won't make you happy but the absence of money would make unhappy many people.
You can try to become Stoic or Buddhist monk but it is not the natural state for most humans.
Happiness comes from within. Pegging your happiness on something external to yourself - material wealth, social standing, another human being - is giving away all agency you have over your own happiness.