Why do we do it? Because we can.
I'm not saying that we should but if you run your own business then you are in a better position to choose when to work than people who work for someone else. Obviously, if you have families - you're more likely NOT to be working at weekends and stick to a more traditional working week (or are you?)
I'm working this weekend - catching up on things I missed or wasn't able to do because my broadband was out all week. That said, I quite often seem to do some work at weekends. Most of my weekend work revolves around reading emails, planning and writing, updating my blogs - more of the fun stuff.
What better way to have work life balance than to spread your working hours throughout the week including the weekend? Weekends are a good time to catch-up or get things done depending how I've spent my week. I'd rather do my shopping in the week than on a Saturday morning along with everyone else, who 'has' to do it then. I do the gardening when the weather is fine - more often during the week. I arrange to meet up with friends for coffee/lunch to coincide with client meetings during the week. I can do non-time limited work in the evening and weekends. I also volunteer on a couple of days a month, which I can also do during the week.
It does depend on your reason for working weekends. If it's because you have to - in order to get all your work done because there just isn't enough time in the week, then you need to look at improving your planning and prioritising what you do.
Don't spend too long working and if you're seeing it as a problem, then it probably is. That's when you need to do something about it.
I'm off out into the garden again ...