The ability to reevaluate your position, thoughts or feelings on something is a very valuable asset to posses. There are quite a few people who have issues with this though. How many problems, in any field or situation you can think of, could be avoided if we'd all be able to say "you know what, i was wrong and i no longer feel the same way about it". Or "well, times have changed and my previous position on this no longer makes sense". Yet there are still a lot of people who are afraid to state that they've changed their mind on something they've made strong statements about in the past. I don't know why that is, though i suspect that in most cases it's all about fear of being perceived as weak. After all, if you're smart and strong you're surely quite capable of forming the "right" opinion initially, so there wouldn't be any reason to actually change your mind about it later, right? Right...
An opinion is always formed within a context. There are many things that can influence or form a context, but a lot of them are temporal in nature. Simply put: things change, as do people (to an extent). The reasons behind an opinion of yours are unlikely to remain the same for the rest of your life. Granted, some of them can remain the same, but the majority of them won't. An opinion that was formed within a certain context is thus likely to become invalid sooner or later within the way you look at things.
Obviously, i don't think there's anything wrong with changing your mind about something. It shows that you've given the issue some new thought. An inability to change your mind, even when the context in which the original opinion was formed has changed, shows a few other possibilities. Perhaps you're unable to assess how the situation has changed. Perhaps you don't want to be perceived as weak for changing your position. Or maybe you haven't even gone through the trouble to think about the issue again. Or maybe you have, and you still stand by your opinion. In the latter case, you can at least rationalize why you're sticking to your opinion. In the former 3 cases, you can't and people will only think less of you because of it.
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