I spent 4 years as a single mother. It was the scariest part of my life. My marriage had collapsed, I had a small child to take care of by myself, with my income only, no child support, in a strange city, in my early 20s, miles away from everyone I knew. And to someone in my situation, Valentine's Day was salt in a big fat gaping wound.
I've been happily married for 15 years now. I never forget how that felt. I take special care to invite single friends to things, and to send cards to widowed people on Valentine's Day. No one should ever feel like I did then... just cut off from humanity, while everyone else passes each other roses.