I Timothy 5:21:
I charge thee before God, and the Lord Jesus Christ, and the elect angels, that thou observe these things without preferring one before another, doing nothing by partiality.
Often as parents, we are drawn to one child or another because of a similarity in interest or personality. There is also usually at least one child that we clash with for the same reasons. (This is usually the one who acts just like us but we don't see that side of ourselves.) But whereas you can choose your friends based on similar interest, you can't choose your children. Each one was chosen for you by God, some to mold and shape you, and some to refine you as by fire! You need each one of them and they need you. They need to know that you love them just as much as their siblings. Nothing causes strife in siblings, like the feeling that they need to fight for your attention and affection. Nothing causes resentment and bitterness like favoritism.
This is not to say that you need to treat all your children exactly alike. My daughter loves Little House on the Prairie and wants me to read it to her every moment of every day. My son wants to read about firemen, but not nearly as long as his sister. He has truck to play with and places to run! There is no partiality in reading different things to each of them, as long as I am reading to both. And I don't have to count minutes and worry that I read a whole chapter to the girls, while only a short picture book to the boys. Now, I'm sure as they get older, things will change and my son may enjoy sitting and listening more, and we will be reading The Swiss Family Robinson chapters instead but that's okay. Meet them where they are at!
When disciplining our children, we have to be fair as well. When I was little, the worse thing in the world for me was having to sit by myself. I hated being alone and having to be quiet in my room was near torture! My husband, on the other hand, loves being alone. Being sent to his room was almost a treat! To one of us, it was stiff discipline, to the other, it was rest. Being fair doesn't always mean doing exactly the same thing.
Like all other things, our attitude is key. Ask the Lord to search your heart and help you see where you are prone to show partiality. Or maybe you already know. Check your heart as you find yourself struggling to be patient with that one child you clash so often with. Make sure you are not showing favoritism to one or the other. Love them all as Jesus loves us.