that is true. a lot of people can be happy anywhere (enviable), and even more people don't know there is anything different (i.e. haven't no comparison). i have no doubt there are plenty of people in all those places that would answer 'yes!' to an 'are you happy?' question.
but this ranking isn't a happiness survey... it's quality of life, and they used some pretty basic and objective measurements. healthcare, debt, wages, reproductive choice access, anti-discrimination protections, etc.
if you're in a position not to have to worry about any of that, or, like some here have expressed, you don't care that others have to worry, of course your QOL is going to be peachy. unfortunately, especially for a state as large as Texas, the vast majority of residents do have to worry about many of those things as part of their daily life. but also...yeah, even some of those people might say 'yeah, i'm happy' bc again, see first paragraph.
this ranking is based on objectively defined criteria and how many citizens of each state are impacted by them.