It's a ubiquitous sentiment that love should always be unconditional, but I have never believed this to be true. Moreover, I believe that love is always conditional, whether we admit it or not. Contrary to what is sounds like, this is actually a good thing.
Love always has certain, basic conditions, mostly being that there is still something recognizable to you in the person you love. If a person changes so vastly that there is nothing left of the person you knew and loved, then it would be completely reasonable and justifiable to stop loving them. People often say to their loved ones, "Nothing could ever change the way I feel about you." Well, that's patently, demonstrably false, and I think we all know it. Obviously something could change the way you feel. If you don't believe that to be true, then you're not using your imagination.
Picture this: as you go to bed, you tell your spouse that you love them, and that nothing could ever change that. Then, an indeterminate amount of time later, you slowly and groggily awaken to darkness, the taste of blood, and a feeling of dull pain. You shake off your sleepiness to realize that you are upright, arms and legs splayed and tied down. You see faint light through cloth as you realize that a bag is over your head. You hear footsteps and suddenly a hand rips the sack off of your head. It's your spouse. "Wakey wakey... eggs 'n' bakey," they say cheerfully. You're in a dark, cold, metal warehouse of some kind, strapped to a large wooden X. Your spouse explains that they drugged you and brought you here for a little fun. Your spouse then directs your attention to three dark shapes in front of you. They appear to be seated people covered in sheets. Your spouse whips the sheets off to reveal your three children, all shackled to the chairs and gagged. Your spouse then proceeds to torture them, and makes you decide which order they die in.
Alright, I'm gonna stop before this goes any further. The question is: do you still love your spouse? Have your feelings about them changed at all? If your feelings are unchanged, and you still love them, then I believe there is something seriously wrong with you. If that scenario wasn't enough to do the trick, then just keep imagining worse and worse things until you find the line in the sand. The point is, it should be trivially easy to find something that would change the way you feel about someone and make you stop loving them.
The condition of love is simply that we recognize a person to be the same person we know and love. I'm not suggesting that people don't change. Of course they do. We're all changing all the time. In fact, staying the same is ill-advised. We should all want to continuously change for the better--to learn more, to grow wiser. But throughout all those changes, we carry with us something that others will recognize--something that makes us... us. What is it that makes us who we are? Though the idea of an unchanging "self" is an illusion, we still have something integral and definitive about us, something that, if lost, would make us cease to be ourselves. So, what is it? To answer that, let us begin removing pieces until we hit the right one. If someone loses a body part, such as a hand, or an arm, or a leg, are they still the same person? Of course they have changed is some regard, but would most of us typically say that they are indeed the same person? What if they lose all of their limbs, but they can still talk and think? Are they still the same person? Yes. So, body integrity can't be it. What if they lose a sense, such as hearing? If someone goes deaf, do we still consider them the same person? Sure. What if they go blind? Again, yes. Even if they go blind and deaf, we would consider them the same person, so long as they can still think like themselves. So, if our identities, what we call our "selves" are not defined by our bodies and our senses, then what's left? Obviously it lies in the brain. What if someone has amnesia, but they still think and act like themselves? So, we're talking about a Bourne Identity case, where someone thinks and acts the same way, but they just don't remember anything about their past. Are they still the same person? I think most would say yes. But what if their brain was damaged in such a way as to alter the way they behave and react to things, to alter the way they think about things. Ah... I think we've hit the nail on the head. A person's real identity, what we call their "self", is the way they act and respond to what they perceive. Sufficiently damage or change a brain so as to change the way a person thinks and acts, and it's safe to say that they are no longer the same person.
A great analogy is to look at a computer (after all, we are but computers made of meat)--if I change a peripheral, like the mouse, is it the same computer? Yes. If I swap out the graphics card, is it the same computer? I would still say yes. If I format the hard drive, erasing all data, is it the same computer? Most definitely. It's the same computer, but with no data. The computer will "act" the same way if you put more data on it. But if I change the processor, is it the same computer? No. Now you've changed the way the computer will respond. You've changed the way it "thinks". Erase a person's brain, and they are still the same person, just starting over from ground zero with new memories. Rewire or damage their brain, and you've got a new and different person who thinks and acts differently.
To circle back, the condition of love is simply that we recognize the person we know--that that "self"--the way that they think and behave--is recognizable to us. If you fell in love with a good, kind person, then in order for that love to endure, that good, kind person has to remain in there somewhere for you to recognize. If a person changes sufficiently by, say, becoming a murderous psychopath, then they're not the same person anymore. The person you love isn't there. I'm not suggesting for a second that we set conditions for love such as expecting perfection. People can change and people can make mistakes without losing our love. The only condition for love is that the person we love still exists neurologically. Of course, if a person's brain is damaged, it's possible to grow to love the new person altogether as well, but it's also reasonable to not love them anymore, as the person you loved no longer exists.
I would also argue that clinging to the sentiment that "nothing could change the way you feel about someone", is morally suspect. If absolutely nothing could change the way you feel, then it seems to me that that would be tantamount to saying that there is no crime and no action so evil as to make you care. They could murder innocents and it wouldn't change how you feel about them. They could burn down a children's hospital and it wouldn't change how you feel. I know that this isn't typically what people are thinking when they say that nothing could make them stop loving someone; it's just a nice thing to say, but I've always tended to be a bit of a contrarian, arguing for interesting philosophical positions. We should all carry with us a sense of good and decency, of love and kindness, and a commitment to those values that supercedes love for any one person. If a loved one were to change drastically enough as to abandon values of good and decency, then we should have the courage to stand our moral ground, to care enough to stop loving them, if need be. Better still would be if we could change them back, but that may not always be possible.
My love for someone would never be unconditional, and I would never pretend that it is. The only condition would be that they are the same good and kind person I've always known them to be, and that they continue to care for me.