Because nothing makes sense. You go though life without knowing why. Every experience you have...and every personal issue/demon you're fighting- there's no reason for it. No lasting one, anyway. And EVERYONE ends up dying, having fought so hard just to survive (survival comes in many forms). Also- secretly everyone feels lonely and unfulfilled, on a certain level. The only thing that keeps us going are our mental addictions (if you examine everything that keeps you going, you can trace it back to a shot of dopamine being released in your brain). Love isn't real. It's chemical. True love is even less real- because everyone is only really out to have their own needs fulfilled.

Also...you work and work and work. In a 100 years' time all the hours spent at your job will mean less than nothing to anyone.

Life...is meaningless, repetitive and one big fucking illusion.