Why Do People Walk Away From Someone They Love?
As defined by our culture, a short answer is that love is necessary, but not sufficient for a healthy relationship that lasts. A friend divorced last year just before his 25th wedding anniversary because, though his wife loved him, she claimed that she could not stay with him and remain healthy. Though it broke her heart (and his, of course), she "had" to leave because she felt suffocated, unseen, and neglected even as she didn't doubt his love for her. This sort of thing is baffling. We may understand people desiring to divorce or leave people because they no longer love them; it's hard for us to get our heads around the idea that someone would leave someone they still actively loved.
That brings us to the long answer. The long answer has to do with why love is not enough to make a relationship work. It's only within the last hundred or so years that human beings even started believing that love should have anything to do with marriage, let alone be sufficient to carry the weight of all marriage is and means. Throughout most human history and in most cultures, marriage was a purely economic arrangement—people married for financial stability and to ensure they had someone to pass down whatever excess was left after their deaths. People may call this way of approaching marriage abusive or oppressive, but it was how things were done in the past. Just as we look at loveless marriages with disbelief and even think that "falling out of love" with someone is a reason for divorcing them, people from the past today would look at our practices with horror. It wasn't that they were distrusting of love; it's that they likely didn't think much of it at all.
Today, we place entirely too much pressure on love relationships and marriage. We think we need a best friend, a financial advisor, a therapist, a defender, and a co-parent all in one person, and we counsel our friends not to settle for anything less. While I don't want anyone to settle (I've tried that, and it does not work at all), I would also like to question if we are better off placing love as paramount than past cultures that didn't emphasize it? Even the question of why someone would walk away from a person they loved belies our unhealthy pedestalizing of this idea of love that we don't even as a culture have a standard definition of. While we intellectually know we shouldn't define love simply by feelings, our actions as a culture show that we really believe otherwise. Although we know we shouldn't place all our hopes and needs and dreams on one person, the fact that so many people still do shows that we still desperately want love to be enough.
Of course, the way we define love matters. If love is that rush of chemicals we get in the first stages of a relationship, that definitely won't be enough to stop someone from walking away once those chemicals subside. Most of us probably define love as more of an action than a feeling; even then, there are circumstances where love is not enough.
The only definition of love I've ever heard that is enough to keep people together but only if both practice it comes from the Bible:
Love is patient and kind; love does not envy or boast; it is not arrogant or rude. It does not insist on its own way; it is not irritable or resentful; it does not rejoice at wrongdoing, but rejoices with the truth. Love bears all things, believes all things, hopes all things, endures all things.
I don't think you have to be a Christian to practice this kind of love, but I don't know any human that can love this way on their own for any length of time, let alone a lifetime. But this, whether Christian or not, would be the only way love is enough to keep people from walking away from someone they love because, by definition—love endures all things—love would keep someone from walking away.
Things get tricky when we bring up abuse. "Love endures all things" has been used to keep people trapped in abusive and otherwise dangerous and damaging relationships. I would argue here that abuse is not love, and while the victim of abuse may walk away from someone they love because they are being abused, the abuser is not acting in love—as in, is not being patient, kind, humble, or bearing all things. It is, in fact, arrogant, irritable, resentful, and does not rejoice with the truth. Abuse, therefore, by definition, is not love. Abuse is one main reason people walk away from someone they love, and the love a victim has for their abuser, while complicating the situation, is not and should not be defined as sufficient to keep them together.
People walk away from someone they love when staying with someone is damaging. In other words, when the love is mutual. While we all long in some way for unconditional love, that's not only not possible, it isn't healthy. That passage above from 2 Corinthians means that love has conditions: it requires kindness and patience and does not demand its own way. It urges you to seek the truth, and it demands that you bear, hope, and endure all things. If the person you love does not practice those things mutually with you, then what you are sharing between you is not love. This passage is often read at weddings, even those not taking place in a church; there is nothing in it or its context that indicates this is only or even primarily about romantic love (between humans). You can still practice endurance, hope, patience, kindness, seeking the truth, and not being irritable, arrogant, or selfish without remaining in a romantic relationship with the one you love.