My mum and I have a firm belief that wonderful as the mother-daughter bond is, we are not best friends, and that mothers and daughters should not describe themselves as best friends.
In part this is because our relationship is much more than that of friends, so to describe it as a friendship diminishes our relationship. But I think it's more than that. I think it's because with friends you have a responsibility not to judge them where as with your children you have a responsibility to judge them.
After all, if a friend is badly behaved then providing you are not the victim your job is to provide hankies and chocolate and fun and analysis and chat and distraction and advice, but only when asked, and if whatever they have done is so bad that you can't do this then you can walk away. However with your children you can never walk away and you have a responsibility not just not to ignore bad behaviour, but to take active steps to make it better, to show disapproval and to set boundaries, all under the blanket of unconditional love.
For although one always hopes a friendship is long term, but often they are not. Some are ephemeral, lasting the duration of a conference or a course or even an evening out. Some last for years but you grow apart, either metaphorically or geographically. Others peak and wane in intensity depending on where you are in life. But the parent-child relationship should not do this. If I am to fulfil my role in our relationship, I will be a constant, not always approving but always there.
What's more, if a friend disappears from your life, you may be sad but you do not define yourself to others, or to yourself, as someone who was once a friend of that person. But once you are a mother (or for that case a sister), you are always a mother, regardless of what events may happen. Even if you left you would remain a mother - it is fact as much as feeling.
So whilst my relationship with my mum has many elements of best friendship about it, I'm proud to say she is not my best friend, she is my mum. And nor do I seek to become my daughter's best friend. Rather I seek to be the best mum I can be. My friendship I will save for those who do not need mothering by me.