Because the universe at its core is being subjected to entropy and decay.
Humans are hurdling through space at millions of miles per hour on a rock that's so big it creates gravity that pulls everything down towards the ground with a strength of ~8lbs of pressure. Our DNA, and thus our bodies, are decaying because of limited resources and being weakened by the strain of frequent replication, while our sun and planet are slowly losing resources, not to mention how the planets of our solar system are being bombarded by space rocks and having their orbits decaying...
Things naturally decay. It's actually a lie that energy can't be destroyed, since the universe is slowly losing energy every day. We're losing heat, things are slowing down, one day the Earth will stop rotating because the oceans being pulled by the moon is causing friction that slows it down by milliseconds each decade.
If we're getting fucked at the fundamentally physical level, then yeah, life is gonna suck. It's all an uphill battle just by how the rules are written, yet alone by how the game is played. I haven't even gotten into how nature is essentially a Hunger Games in and of itself.