@Dossema
IMO there are many reasons all different from person to person...... but so many seem to think "no it just happens" ...i do not believe that.
I am not justifying or making excuses. just writing out from experience in witnessing in real life
People meet someone. maybe a friend or co-worker or anyone .. When they start to talk maybe make a connection........they are filling a need or want not being filling in their relationship .
Most (IMO) if not all start innocent enough not out "shopping" for someone. A connection can just happen but that was not the plan in talking with someone.
If a person makes you feel attractive ...it feels good .. I know I had someone notice outfits, hair changes and my spouse says nothing....... no compliments .....no "I like your hair that way" type things . Maybe they feel listened to ......not just a head nod or no response from partner ...
spouse buys his favorite things and claims they are my favorite too ....which makes me just an extension of him not a person that has my own preferences or favorites? OR i am not important enough to care what my favorites are. or my opinion on anything.
Different ideas of what they want in life........ if one partner is OK just sitting in front of a TV and other wants to do things outside of house.
I think as we get older intimacy can become an issue as some lose any interest and just expect their partner to be OK with that. no discussion just done. maybe someone let themselves go and are no longer attractive to their partner.
I could see how a simple friendship could become more especially if the other person is missing things in their life too.
Finding out the real reason and really discussing it seems from my perspective the way to get through it. Pretending and hoping things go back to way it was is unrealistic... some people fall out of love.... so many reasons. I was cheated on a LONG time ago and I saw at the time. I was a really horrible person to get along with. We worked through it .....and I saw and accepted my part of our relationship failing .... now tables are turned I understand better how it could happen.