Because when someone is dead you can no longer talk with them, be with them or share an experience with them. Ofc that is sad.

The only reason I don't kill myself is because I do not want to transfer my suffering to my family: it would be selfish because I can imagine myself in their position - I would be extremely sad if some of my family members comitted suicide.
So I'll wait till my parents are dead and until my brother has a stable life - then I'll finish myself for sure since I'm incompatible with this place. To me, that is one of the few acceptable circumstances where it is "ok" to commit suicide: when the pain you transfer to your surroundings is minimal.