I think as others have said, it's a lot to do with contrast. After being physically ill, I'm much more grateful for life in general. Just the relief of not feeling ill.
Overall though, it's hard to enjoy life when deep down, you resent and hate being alive. When that is your foundation then, everything on top is an annoyance to varying degrees and- there's a whole lot of chores and work that have to be done in order to sustain the life you don't want in the first place!
So much of the time is spent thinking: Why am I bothering to do this? The answer- because you're ensuring your life doesn't get any worse. But, why am I bothering to live at all if I don't really want to? You're doing it so as not to upset the very people who brought you into this mess and expected you to thrive in it. (Parents.) It's hard to be grateful for something you don't want and aren't really permitted (freely) to quit.