How do you fall out of love with someone?

You don't. If your really desparate though, try to find things that you don't like about him. See how he is with your feelings. If your dating though, please please don't be hard on them. It's a rough spot to be in
You either love someone or you don't.  Sometimes to one mate it doesn't seem to matter how their mate treats them and it can even be an abusive relationship.    A person could be having an affair with a married man and generally the man (or woman who is married) can be self-centered and not care about the affair at all and is leading the person down the garden path.  There are many different scenarios. This is a twisted sense of love.  Love is caring about each other, honesty, loyalty, communication and the feeling you can tell each other anything and can beat any problems in the world if you're together. If your mate doesn't have these factor then it's simply not love and you're brainwashing yourself into thinking it is.