3. You find out that he or she is a totally different person.
You have slowly noticed the changes in his or her attitude as days, months and even years passed by. He or she turns out to be not the same person that you fall in love with. He or she has become so different in a way that you can no longer connect with each other. The sparks are simply not there anymore. Bottom line is, before engaging in a relationship, you have to know the person very well first.