Flickr / Vladimir PustovitHere's the thing about relationships. Sometimes they're candlelit date nights, walks through grassy meadows, and feeding each other cake at your wedding. But sometimes they're doing the dishes, comforting a screaming baby, and fighting over whose turn it is to do the dishes or comfort the screaming baby. Which is to say: While sometimes your love and passion for each other is obvious, sometimes it's really, really not. So how do you know if your relationship is terrific — or heading south? Psychologists have spent years studying the traits that are fundamental to successful long-term relationships and have come up with a few key ideas.