Because you are alienated from the time and energy you put into your work; because it is siphoned off by some big-wig share holder who couldn't care if you lived or died, let alone what your dreams and aspirations are.
And, no matter where you work, the system as a whole is built around your alienation and impoverishment with greater or lesser severity.
You're unhappy because capitalism is built off of your back and kicks you out the window as thanks.
"The alienation of the worker in his product means not only that his labor becomes an object, an external existence, but that it exists outside him, independently, as something alien to him, and that it becomes a power on its own confronting him. It means that the life which he has conferred on the object confronts him as something hostile and alien."
https://www.marxists.org/archive/marx/works/1844/manuscripts/labour.htm