I agree if you're working in some drone farm. Without a personal stake, why do people feel enthusiastic about following orders that keep them locked up at work all day? I understand that there may be coercion, like the threat of being fired, but if you're just writing software for your company, where does the enthusiasm for long hours come in? I'm freelance, and I love working on my projects. I often work long hours. I hardly stop. But I have a 100% stake in what I do and generally choose projects I believe in, like endangered language preservation or stuff that I find intellectually stimulating. Why would someone want to work at Microsoft or Google past 6pm? Why do people give their souls to hives?
Even if you're not working at some drone farm you should probably take time to yourself to relax and spend time with your friends/family. I understand if you're freelance and you have deadlines and whatnot you'll need to work long/odd hours, but honestly you shouldn't have to kill yourself with work. I enjoy what I do and have a large equity stake in it, but I'm not going to burn myself out because "I love what I'm doing." It's not good for me, it's not good for my family, and it's not good for my work. If there is a tight, important deadline sure I'll work longer hours, but in most cases it's almost certainly unnecessary and definitely negative.
Or, more generally, there is more to life than one single thing. Some people let a single thing (or a very, very small number of things) define themselves, and it's a bad idea.
Think critically - how are you defined? If you asked the 5 people closest to you, how would they describe you? If they can't get much further than "good software guy", be careful.
Without some drive and enthusiasm about what you are doing those 9-5s could end up being a lot worse than the alternative of putting in some extra effort because you are really enthusiastic about what you are doing.
