I have seen it so many times, in my own life and those who are close to me. You have a decision to make and you are 'umming' and 'ahhing' over it for days, weeks, sometimes months. You go over the pros and cons in your mind and just can not find a way to answer definitively what it is you are going to do.
I propose that often the real reason is because not only is there emotion involved but, and this is the sucker punch, because you know that if you really follow your heart of hearts someone you care about is going to get hurt. You are a good person, you don't want to cause any pain or suffering, you don't want to disappoint, you don't want to back out of a promise or cost someone money or time that they have invested in you or because of you. Perhaps it is you that you are trying to keep safe and you are scared of the risk.