I have often thought the same.
Some friends stay and some go.
I truly believe all friends are sent to test us,to teach us lessons through life.
How we respond to them and how they respond to us.
We learn which ones to trust and ones we grow to distrust.
I say 'distrust' because sometimes a friendship may grow and look promising but then they may make a comment which for some reason,gives me a peep into their true nature.
Whether it is a comment made in humour or serious vein,I see the true person hidden inside.
One part of nature I cannot bear,is sarcasm.
That decides me whether that friend is to be a friend for life.

So yes,I think certain people are sent to us to test us and to learn how we should conduct ourselves in friendships also.