The reason why people feel the need to justify it is due to the fact that when you announce things like that, even breaking up with actual people, people always ask the same question, "Why?"
From there, you feel the need to explain your reasoning, on top of another reasoning. There is always "I did it for this reason." Then on top of that, people say, "But *insert defense, or positive attribute here*". Then you have to defend your reasoning with another reason, or why you may agree, or disagree with the others original comment, then the cycle continues.
Others may not feel the need to ask, defend, or feel as if people do not have to justify their behavior, but some people do and others feel they need to explain. I can understand, as being the fact that I have always been a fan of Disney and while I still love Disney, people still felt the need to be shocked and ask, "Why?!" When I told people I wanted to "take a break" from Disney and get an AP from Universal again.
At the end of the day, a lot of things (in general) have pros and cons. A lot of people discuss the pros, but do not point out the cons, or the flaws and sometimes people need to have that in check. Some people find that enlightening and some people find their bubble has been burst. Which is why these "posts" are controversial, because whether people agree, or disagree, everyone reacts differently.