1. Stop doing the little things.
If you ever want your relationship to spiral into a big ball of sadness, you need to stop doing the little things for your significant other. Stop buying her flowers. Stop hiding love notes around the house for him. Stop going to the movies. Stop making time for each other. Stop saying "I love you" before hanging up the phone. Most importantly, stop putting any effort into making each other happy.
On a less sarcastic note, not many of us want our relationships to be ruined. I can't stress the importance of giving your "other" just the slightest bit of attention. It really does go a long way. Some of us feel like we are too needy and we just ask for too much all the time. That's draining to our person. Some of us feel like we ask for so little and just don't receive the right amount of love. That's draining to ourselves. No matter what your situation may be, too much attention or too little attention, you need to pay something to find out what they need from you. It takes a few seconds to ask the simple question: Are you OK?
Relationships sometimes fall into a weird state where you just don't know what to do about a certain issue. It's important to remember here that ALL relationships are different and we all deal with problems in different ways. Some people choose to fix their problems. Some people choose to break up. Some people are just stuck and stop putting the effort in to it and just see where it goes from there. I don't have an opinion on the first two options, but the last one is never the way to go, and so very often that's the one we choose to do.
Even if you are in a relationship that you don't do cute and loving things for each other, you still have ways of showing your love. Do them. I was told numerous times that I live in a fantasy world that I think all relationships should be peaches and cream and happy-go-lucky. I don't believe that at all. All I believe that is the relationship should always have a certain amount of effort pumping into them to keep them alive and thriving. If you aren't putting anything into it, why are you still there?