Do you like flat characters?
How about cliché ones? Or cookie-cutter ones? Doubtful.
We want to connect with the people we read about. They need to be real to us. They need as much depth as a real person.
When creating your characters, it is always important to make them unique. Many authors take this advice and give their characters tragic back stories. However, this can become a writing crutch. This trend is reinforced by Hollywood, which has embraced a similar philosophy.
The problem with tragic back stories comes when an author makes that the character's defining/only characteristic. It should be one key ingredient to their personality, but it is not all they are. They have an entire history behind them, not just that one event, although it may certainly be a major component.
The back story forms your character, affects their decision making, their daily lives. Show that affected behavior. If a character has lost their spouse, they may be unwilling to commit to another relationship, or may treat potential romantic partners unnaturally. Remember to realistically portray the balance of that tragedy and all other factors in their past.
Next time you compose a tragedy for your character's past, remember that he or she is like any of us: they may have suffered, but they are made of a lifetime of experiences. Use and show those.