Friendship Fleeting

When I was 5 I met my best friend. And like all little girls that age, I thought she would be my best friend forever. We had ups and downs growing up like all friendships do but somehow we got through it all. She was a bridesmaid at my wedding and I was one at hers. She was one of the first 5 people that I told I was pregnant with Kai. We spent many nights laughing and crying together. I just knew in my heart that I would always have her in my life. Sadly, things happened and things were said on both sides that can't be erased and we no longer talk. I am deeply saddened by this. Soon after the death of this friendship my mom told me that people come into your life for a reason and when they are no longer needed for that reason then they leave your life. I now wonder what her purpose in my life was. The loss of our friendship has been the hardest break-up I have ever gone through. There was never any closure so I am left with just a huge hole in my heart. So my question is, if friendships really are fleeting then why waste the time in letting someone in? I guess that is a question that I may never know the answer to.