It is Texas pride based on the fact that Texas was at one time an independent country and even now is an economic engine that produces goods and services of greater value than several other US states put together.
It can also be viewed as a veiled threat as there are many people who still want Texas to succeed and would love to see that happen. Just look at the number of times people in the Texas Legislature have called for succession. That says a lot about Texas and its strong conservative culture.
As generations change and more people move into Texas the culture is changing and not always for the better. Forty years ago, this was a big saying. It is hardly mentioned anymore due mostly to the changing culture.
Times change, people and attitudes change with time too.
