One of the saddest causes (and I would say common) that can end your friendship with a close friend or even a member of your family can be his/her marriage.
Imagin you have a very close friend who is single. You are like a brother or sister. You grow up together. You have lots of things in common. You share precious memoirs. One day this very dear friend get married. You can't wait to meet the girl/guy. Finally you meet "the significant other". First meet you can feel something is not quit right. You can feel the slight tension at "hello". You deny it.
This friend of yours is so dear you can find a way to like his/her love of life. But life has its own way. Gradually you notice your friend is changing. You are still in denial. You don't want to accept that your friendship will never be the same. This new "significant other" turns out to be control freak. Its her/his way or highway. and your friend has to get along and the price is your friendship. Gradually you feel the distance. Gradually you watch your friend leaving your book of life. No matter how loud you cry for him/her train of life taking him/her away from you. And with that, a piece of your heart goes. Your heart aches but there is nothing you can do.
It doesn't have to be this way. Sometimes when your close friend gets married , things get even better. Your friend spouse himself/herself becomes another best friend of yours. When my best friend married her husband became my best friend in a bit. The three of us had a lot in common. We enjoyed talking to each other till midnight without noticing a clock.
Dear spouses, when you get married please don't try to cut your partners friends and families ASAP. If they were doing fine before you appeared in their life, trust me they don't need you to coach them how to manage their 20-year old friendship. Why do you think you have a right to suddenly take everything over. When you get married you don't own your partner you are his/her PARTNER not boss, not coach, not.... You two are supposed to share your life together not taking over one another's.