Yes it's very sad when you lose a friend I mean it's a natural thing to feel sad after that. I know what kind of feeling that is, I have lost few of my good friends over the years. I cant say that I was the reason, I even don't know how that happened. Maybe because I have changed my place of living and we are far away.
I'll mention just one case. When I saw my old friend after 4 years I hardly recognized him, not because his looks but because his behavior. He changed to a person that I didn't like at all. So naturally I couldn't stand him at all and that was the moment our friendship broke. Also sometimes you even don't know who truly are your friend's.
You must be careful there are many that can pretend to be your friends but when you see their true face you hardly recognize them. I know that losing a friend is a bad thing, but sometimes there is no other option than just that. As I sad I lost few of my friends and I hope that doesn't repeat anymore.