Just another bent on things from me here. Somewhat of a personal testimony and witness on my part in order to help to answer rockerchick_682's question.
"Why do people fall out of love?" A deep question and worthy of discussion. Asking such a question would warrant discourse as to what love as well as friendship are, in terms of the contrasts and similarities between the two, and the definition of the state of being at which one finds oneself to be involved in such with another person in either state of being with another.
I don't know that people really fall out of love. In my own personal experience, I have had numerous "loves" with women in my life. Did I ever quit loving some of them? No. Even though I still love some of them, did I choose to move on with my life and grow to love another? Yes. Did I truly love every one of them? No. Did some of them do things in their lives and in our relationship that would make me think twice about continuing a relationship with them? Yes. Did I do things in my life and in our relationship that would make them think twice about continuing a relationship with me? Yes.
Am I still on speaking terms with some of them, and to the extent that we could consider us "friends" with each other? Yes. (Getting into who are your real "friends" are as compared to who your "really good acquaintances are" is very much along the same lines as trying to define what "love" is.)
Yours is a really tough question to answer as well as to discuss. I don't think that there is any set answer.
I'm not going to get into what I think love is, because that would open up a another whole discussion. But, I do want to present a case in point from my own life that might help guide you into understand and assimilating an answer for your own life.
Several decades ago, when I was in college and dating a number of women, there were just a few that I could say with whom I was in love. For me, love between persons, is a very deep, personal thing that goes beyond any simple definition. It is not just something that is physical, but something that also involves one's mind and soul as well as heart.
One of the women and I decided to get married. Did I not love the other two? Yes, I did love them. Did the love between I and each of the other two fall? No. But, circumstances being that they were, I married one of the three. I even invited both of the other women to the wedding. One of them showed up. Some years later, when I saw the one that didn't come to the wedding, I found that she was happily married and living a good life. She said to me, that even though she was happily married, that she did not have the deep kinds of meaningful discussions with her husband like the ones that she and I would share. Was her not having the same kinds of discussions with her husband like she and I had shared something that would be a missing factor in her marriage? No. Did we still love each other? I think so. But, our getting married might not have been a good "fit" in other ways. There were factors in her marriage, other than deep discussions, that made their marriage whole, with body and substance such as to make it sound.
I like and can appreciate all of the answers above. They are based upon each individual's understanding and experience as well as encounters with others. But, I really like what Rockabilly1955mama in particular had to say::
For many personal reasons. Things change, people change. Some for better, others for worse.
It's hard to answer this question because each circumstance for "falling out of love" has it's own reasons.
I like the simplicity of her answer. Even though it is simple, it speaks volumes.
The reasons that people "fall out of love" can be as varied and many as there are differences in each snowflake that falls. When you think that you have fallen out of love, it is best to cherish the memories of the good things that happened, live with the knowledge that you have gained by "falling our of love," and to just move on with your life. Maybe you will be friends with that person, and maybe you will never meet again. Only time and circumstances will tell.
And, the older you get, the more reasons you will find there to be for "why people fall out of love."