Suddenly, the person who always completed you, who always finished your sentences, is gone. Your friend always knew what you were thinking without you having to say anything, so finding the right words to speak with someone else? It's impossible.
And when you see that yes, your friend is still alive and well and seems to be doing just fine, the jealousy hits. You'll go green with envy, because why does your friend deserve to be okay when your life is falling apart?
Then, one day, you won't hurt so much. You'll meet some new friends, and maybe you'll click with one of them like you clicked with your previous best friend. It won't be the same, but isn't that what's great about life? It's always a new adventure.