See, here’s the thing; there are grains of truth in that notion. Certainly, it is NOT possible to be happy unless you make the decision that you want to be. There absolutely ARE choice points where you can make decisions about how to react to the actions of others. But I’ll never understand how that is supposed to translate into the thinking that no outside force can ever contribute to a person’s unhappiness, and failure to be happy is a character flaw.
Clinical depression exists. Also, people can come up to you and do you violence, which may result in your being unhappy. People can come up to you and do you *emotional* violence, and if you are not yet a fucking Bodhisattva, that can sometimes cause you to be unhappy too, especially when the perpetrator is someone you trust. When those choice points come around, you can’t always get into the right head space right away and avoid being thrown off course. At least I can’t, and I know I am not the only person in the world who can’t.
Despite pretty severe, treatment resistant depression, I am generally a pretty happy person. But I have bad days, like anyone. Shantideva said “Putting up with little cares, I'll train myself to bear with great adversity” and I try to live that. It helps a lot. But it isn’t a fast-acting panacea, and… I have bad days.
But I am damned if I’ll let anyone tell me that I’m bad or weak because I suffer, just like all sentient beings do.