Yes, this can happen to anyone. Most of us realize that we are not with the right people after we have known more details about them. You are required to inform the person of how you feel to avoid any hurting breakups in future.
Yes, people change so much over time. Think of all the change in yourself from the age of 10 to 15, or from 15 to 20. When in a couple, if one person is moving ahead in their work or education and the other is doing nothing but watching TV, love will lessen. If one wants children and the other doesn't, love will lessen. If one needs constant attention and the other is a quiet, private person, the match will not benefit both for very long.