Question about cheating spouses...?

Could you still be in love with someone and not know it? Say for instance a husband, who always seemed very loving and happy, started to change over the course of a couple months, becoming distant and paranoid, and you found out that he had developed feelings for a(married) co-worker that he barely knows(and I mean he knows NOTHING about her and sees no fault in her) and he left, saying that he loved her and hadn't loved you in years. Now obviously people will say 'happy people don't cheat" and that may be true, but could the spouse who has developed feelings for another be infatuated with them(or how they make them feel about themselves) and confuse these strong feelings for real, mature love? I've heard people say that being infatuated with someone can cause a "high". Something about dopamine in the brain, or whatever... My question, which is probably pretty obvious that I am talking about my own personal issues, is that if you now someone well enough to know when they are changing right in front of you, know that they are hiding something, wouldn't you have known if they hadn't loved you in years? All that emotion faked? All those experiences between you... I just can't buy it... Is it possible that he is burying how he feels about me because of the "high" he is getting? To make his departure easier? Or is he saying it to possibly make me focus on what he is saying instead of what he is doing, cause while the cheating sucks, telling me that the love has been gone for years is the one that really hurts...