This is true! becoz a best friend is always a best friend in whom we trust and share. But when relationship changes to lifepartner, the trust, love and other essence will also tend to change.
Secondly we can share all (all means ALL) things with your best friend, but when he or she becomes your life partner, we cant share everything as we need to maintain a healthy relationship we tend to hide few things.
Finally, as the days go by and in the process of hiding few things, one tend to search for another best friend to share, which lead to relationship break down. Also the existing partner may feel that my best friend turned a life partner is not as good as before, and thus causing bitterness in Life.
