The reason that people break up are almost legion. As Leo Tolstoy said in his vast and sweaping treatise on family life, Anna Karenina, (matter of fact, it starts it off, its the first line of Chapter 1);
Happy families are all alike; every unhappy family is unhappy in its own way
Sometimes people just grow apart, sometimes one grows and the other doesn't, sometime they don't grow and expected to.
If one person, the TS, has a right to say "this is not my life, I want another" then who could blame the other in that relationship for taking upon them the self-same right?
As for why... people who never have experienced something always seem to know the most about it?
One of my favorite quotes, (ascribed to noted American philosopher, Yogi Berra) time tested and battle proven is that:
In theory, there is no difference between theory and practice; in practice, there is.
By which Yogi was saying its real easy when you are sitting in the stands eating popcorn to play the game, its a lot harder behind the plate.