When relationships are new, we only see the good and wonderful things about that person. But as the relationship matures, we tend to become less infatuated. Now the work begins.
We have to accept people for who they ARE, not who we THOUGHT they were, just as we want them to accept us. We have to accept their faults and work around them, just as we want them to accept ours. Don't try to change anyone because the truth is, the only person you can change is yourself. Don't dwell on the things about them that irritate you. That will only magnify them - make them seem bigger than they really are. Keep looking for their good points. Everyone has them. Remember what attracted you to that person in the first place in the case of a romantic or friendship relationship. Look for everyone's good qualities and amplify THAT. If you slip up -- and being human, you probably will -- just pick yourself up and start over again.
Like I said, it takes work and it's not always easy. But if it's a relationship you want to hang onto, it will be worth the effort.