Because is the only thing we have, and then death, which scares almost everyone, me included, so you keep going and you have moments where you break but not literally, you yell at that one person, or stop caring about yourself and others, you do things that harm yourself and/or others...
I feel that people that enjoy life just cope with the agony of it turning a blind eye on things, they find something that makes them happy and go for it, the world is full of pain and anxiety and we have to do all the work ourselves to get enjoyment out of something here, people like us are just too tired or too deep into the abyss to look for anything, so we exist day by day, crying, being angry, being miserable, until the day we die either by our own hands, other people's hands, or naturally. Look at someone """"happy"""" and try to analyze with an open mind their life.