Focusing on the things we love and appreciate about each other, rather than the things we aren't so crazy about, is probably the most important thing you can do to keep your relationship strong and happy.
While the idea is certainly true for both partners, from what I have seen and experienced in my own life, it is particularly something that women succumb to. I don't know why, but it seems to be female nature - maybe it goes all the way back to "the fall" in Genesis, where God says, "you will desire to control your husband." :-)
Keep in mind I am speaking in generalities here - I'm sure there are men who would like to change things about their partners, but interestingly enough, while men are known as "fixers" when it comes to physical things, women are much likely to want to "fix" things about their mates. Don't get me wrong, there is absolutely nothing wrong with wanting your beloved to be the best that he or she can be. But it's a slippery slope that, if we're not careful, can lead to dissatisfaction, resentment, and eventually even the end of a relationship.
If you find yourself thinking this - especially this last thought - you're in the danger zone. Once you start thinking that some other man might be better for you than your own partner, the end isn't far off. This is what leads to affairs, divorces, and destroyed families.
But you can stop it well before you ever get to that point. You just need to watch your thoughts, and be aware of the sneaky little desire to "fix him." Be aware of this thought, as it will crop up at some point, regardless of how strong your relationship may be. And when you see it, notice it, and then send it on it's merry way.
How? It's simple, although not always easy...
As any success guru will tell you, "two contradictory thoughts cannot occupy the same space." The way to stop focusing on those pesky little things you want to change about your partner, is instead to focus on all the wonderful things you love about him (or her). When you catch yourself starting to dwell on his/her flaws, stop for a moment, and turn the tide of thoughts in the other direction.
It can be as simple as, "Man, I wish he would pick up his socks and put them in the hamper! He's so messy and I'm so tired of picking up after him.... But...I do love how he rubs my head when I have a headache, and how he always listens to me complain about my boss, and has a glass of wine ready for me when I get home from work. I'm really lucky to have a man like that!"
Whatever it is that you love about your partner, just stop and think about that for a few minutes, and I bet it will already bring a smile to your face! It may be hard at first, especially if you've been grumbling about him under your breath for a while, and if things have gotten so negative that you have wondered if you even want to continue the relationship, maybe he's really not the right man for you. But try to remember all the things you love about him, and the reasons you are together, and see if there is something worth sticking around for. And if there is, then you owe to yourself, and to your partner, to make it the best and happiest relationship it can be. And it's just not going to be that if you focus on what's wrong with him or her instead of what's right.
It may sound simplistic, but I can tell you from my own experience that this really works. I have gone from being unhappy, frustrated, and dissatisfied in a relationship - solely because of my expectations of what I thought he should or shouldn't do - to blissfully happy, loving, and highly satisfied with my relationship and my man, all in the space of a few days just by changing my focus. This has happened in my life more than once, and I know other women who have experienced this as well.
Just remember that old maxim - you can't always control what happens to you, but you can control how you react to it. Apply this line of thinking to your relationship. Remember - negativity will never lead to a positive outcome. Take responsibility for your own happiness, and celebrate the wonderful qualities in your partner that make him the one you want to be with! Celebrate, appreciate, and enjoy those things to the utmost, and let the rest go - they're not important in the grand scheme of things anyway. What's important is the love you have for each other, and showing that to each other each and every day.
Now go tell your loved one something specific that you love about them, and make their day! :-)