Some people have a great relationship that lasts all their lives. Other people never seem to have a relationship that lasts more than a few months. Somewhere inbetween are the people that have a great relationship that slowly fades away. What is it that kills this last kind of relationship?
You may have heard of the seven year itch. It's a cultural reference to the fact that after a few years, and seven is NOT a magic number, a relationship is no longer as exciting as it once was. In fact, this can, and does, happen after a few months... but there is another drop in excitement, after this initial one, when you have been together for a few years. So why does this happen?
The initial drop in excitement, after a few months, is very nearly inevitable... that initial excitement is due, in large part, to the newness of the relationship. You are discovering all sorts of new things about this person who is so important in your life, thinking about them constantly. They are never far from your mind.
Once you learn the basics of who they are, which takes a few months usually, the excitement drops. You can then, if your relationship has decent foundations, maintain the new level of interest and excitement for a few years, at which point you know far more than the basics... in fact, you may feel like you know pretty much everything about your significant other (chances are pretty high that you don't, but that's a topic in itself). So, once you arrive at this point, the excitement level drops again. That's when the seven year itch kicks in to high gear.
Both of these drops in excitement have something in common. Both of them occur when you feel like there isn't as much left to learn about the person. This is NOT, however, the actual cause of the drop in excitement (or the drop in closeness that often accompanies the drop in excitement). It is only a more visible part of a deeper issue.
The real killer of good relationships, and one that often keeps them from cementing in the first place, is your partner dropping out of your conscious awareness. When you reach that first point, where you feel like you know the basics of who they are, you stop thinking about them quite as much. You stop thinking, or at least stop thinking it as often, "I wonder what she'd think about this?" or "I wonder what she's doing right now?". Then you hit that second drop when you feel like you know your significant other very well, and you feel like you can actually answer the questions in the previous sentence with a fair degree of certainty.
The killer of good relationships is familiarity. When something becomes familiar, our conscious mind tends to start handing it off to the subconscious to deal with. You can choose otherwise, but it has to be an active choice... the default is to pass it on and stop being aware of it. This includes people and our relationships with them. When you stop being aware of someone, you stop thinking about them, you stop giving them (and their happiness) your attention, and things deteriorate from there. And once this happens, you look at your relationship, when it comes to your awareness, and realize how far it is from what it once was. That makes you want to think about it even less, because it makes you feel bad (sadness, guilt, anger maybe... mental anguish). So it becomes a vicious cycle that takes a conscious effort to break.
You can recapture the excitement, though, and the depth and closeness of the relationship. All you have to do is start thinking about her more often (and getting her to think of you!). There are lots of ways to do this, but the biggest one is to do something different. Something you haven't done before, or haven't done for a long time. Or maybe even something that you have done, but you've never done together before. You can also come up with surprises (little ones... don't scare the snot out of her) to make her think of you... leaving her a note, making her something, going shopping with her specifically to buy her something (it doesn't always have to be about you!). The more unusual, the more new it is, the more she will think about it, and since it was from (or with) you, the more she will think about you. And you will obviously be thinking of her, because you'll be planning out what you can do next.
So... if your relationship isn't what it once was, don't give up on it. Instead, think of things that you can do for your significant other, or better yet, things you can do WITH your significant other. Go to a new restaurant together, go to the place where you first met, go have a picnic in the park (or on the beach... depending on where you live). Do something you don't normally do... and start doing it more often. But don't make a habit of it, do it consciously!