Feeling pressured to work beyond the standard 35 to 40-hour work week is one primary reason that people have a poor work-life balance. After all, if there's less time for your personal life, then you'll struggle to strike an even balance between the two aspects.
This feeling may not even be due to longer hours. Having a long commute to and from work can still instill a feeling of obligation to the job, instead of giving you time to unwind after work.
Being unable to shut off communications with bosses and co-workers outside of working hours can also contribute to poor work-life balance.