Because they would get bored. Because they would turn their backs. Because they wouldn't understand.
I talk only to my BFF about my dark thoughts, and even he doesn't know everything. I can sit in my bed for hour and crying like a little baby without no specific reason. How can you tell anybody, what's the trouble, when even you don't know what's going on?
And that's the moment when even closest friends got tired. You're sad all days, they wanna have fun. First they stop chat with you on daily basis, then they stop to invite you to events, because you're always the one making sad faces.
Then it hits you. "I'm actually alone in this." You still don't know what's making you so sad, but the thoughts are getting more dangerous. They you realize, that you actually don't care what's gonna happen next.
People around you have their own lives, their own troubles, and you don't wanna add up. You don't want to bother anyone. Not even your closest and best friends. Because it's not fair from you. They deserve to be happy, and if not-telling them my sorrows will help them achieve happiness, you will go for it.
'Cause that's what best friends do. 'Cause this isn't about you. There is actually no one, who cares enough, to see through your fake smile, laugh and words. You are alone.