Something I learned back when I started my first company is that there is always more you can do. There's always one more email, one more phone call, one more Google search that might help your business.
In the end, however, each day has only 24 hours, and when those hours are gone, they are gone forever. Moreover, you have only so many days to live, and each day that passes brings you one step closer to the cold embrace of death.
Starting companies (or investing in them) is hard work. But stress, however justified, does not help your productivity. It's better to compartmentalize and make sure that you leave enough of your time and energy to actually live your life.
The best way to find happiness is not to defer it to a far-off, continually receding future, but to seek it in the here and now.
(originally posted as a comment on Brad Feld's blog)