Because it spares you emotional pain.
For years, I was, some how, able to deny reality and lie to myself - "Everythings fine. There's nothing wrong with what you're seeing - carry on as usual." This allowed me to appreciate the comforts of everyday routine, the security of the familiar, and the tranquility of repetition. I was able to enjoy that as much as any other person. But as time went one and I got older, the enunciation of the truth became less and less ignorable, until I could deny it no longer.
One day while I was getting a common punching session from my dad, a realization hit me like a ton of bricks - "There is something terribly wrong with my life....isn't there?"
I had been denying that all my life to spare myself emotional pain. I knew that deep down in my subconscious - but I kept it a secret, even from myself, because I, somehow, knew at the time, that I couldn't handle the situation I was in.