I'm trying to read through all of these replies, but I've seen mention of unconditional love a few times and I want to respond.
To those of you that think you have unconditional love with your SO, think again. Do you know what unconditional love is? I doubt you do. It means that no matter what your SO does, you will still love them the same.
To those of you that say you have unconditional love with your SO, would you love them the same if they cheated on you? What if they constantly put you down? What if they made you choose between them or your family? Unconditional love isn't the same as being in love. Unconditional love is a falsehood which doesn't exist in my opinion. There will always be SOMETHING that can make you not love another person.