Dealing with Grief: Losing Someone Close to Your Heart

A friend that I've known since I was a baby (he was my parents' age, they met him in highschool) passed away from cancer recently, a week before my birthday, and his funeral was the day before my birthday. I don't like to make it about me because he is the one who suffered and died and I can't even imagine what his wife is going through but I've never had to mourn for someone so close to my heart before. I was already doing poorly mentally and now this. And the only thing that is truly certain in life is that I will experience this again but worse as time goes and people I'm even closer to pass. It's cruel to have been born into this, and I can't understand why people elect to have children who will also suffer much if not all of their life.