What in Texas’ history accounts for this continued mentality of Texas as a thoroughly unique place within the United States, both socially and culturally?

For many years, the official tourism motto for Texas was “It’s like a whole other country” and for many Texans this remains an unofficial credo for Texas. Texans have traditionally defined themselves first and foremost as Texans, moreso than any other Americans have identified as citizens of their state since at least the American Civil War. What in Texas’ history accounts for this continued mentality of Texas as a thoroughly unique place within the United States, both socially and culturally? Do you find this pervasive conception of Texas’ uniqueness to be justified based upon what you have learned?