I think happiness is definitely defined by both internal and external factors- and often the two can play off of each other.
To a certain degree, we all depend on each other for our own happiness, the herd mentality often means you're not going to be happy when someone you love is unhappy or suffering, but this is also seen in a billion self-care posts on the internet as a bad thing
depending on others for happiness is cruel to them and yourself!!11! blah blah blah but I think that's a rather unrealistic ask of mankind to directly defy their nature. Sure it's super great if you can just be happy by yourself but there's a load of factors that are sometimes out of our control. Kat mentioned mental illnesses- quite literally chemical limitations to someone's happiness, and it's not because they chose not to be happy -I don't think I've encountered anyone with mental illnesses who wants them- they simply lack the control over their own happiness.
A lot of times people would turn to religion- to answer those unanswerable questions, like the disparity in control over happiness, some higher power or being is testing people or is going to ultimately bring everyone and everything to the ultimate prize. In my opinion that just falls apart the moment you look at how unjust of a system that is, some people
can control their happiness; they have the confidence, the time and most likely the money to do so, compare those people to say the ones born into wars, thrown into domestic violence situations and facing oppression. What loving God(s) could put their people through such hardships, with no very limited control over their own happiness whilst let others have free reign to determine theirs?
There's also the question of
what actually is happiness? Is it purely the chemical release of serotonin, dopamine, oxytocin and endorphins into your brain? Is it a bit more wishy-washy than that? I don't think you can truly define happiness at all- it's a subjective concept, no two people will experience happy the exact same way, that makes it pretty hard to tell if you can control it or not, I might be happiest when I'm talking to the people I love, for someone else they might experience the same level of happiness from eating icecream or petting a dog. You can't measure something you can't define- although without getting too ethics-nerdy, I'm sure Bentham and Mill would say pleasure is equal to happiness, and the whole hedonic calculus comes into play and that just falls apart entirely for a whole host of reasons!
To summarise I suppose I disagree with the idea that we can control our own happiness, I don't think it's entirely possible for any singular thing to dictate an individuals happiness, there are a lot of physical, geographical, economic and mental factors that contribute to someone's mood. I'm also not certain the question can be answered- happiness is too broad of a spectrum for it to be talked about easily.
My apologies for the length,, I haven't been in one of my philosophy and ethics classes for about two months and I miss it dearly, got a little carried away! I look forward to more Ash