NO healthy relationship is without conflict. You have to first understand that before delving deeper in the question you pose
Relationships require honesty and unless you're dating someone who is identical to you in every sense, you will experience varying degrees of conflict; some more than others. Mature people can find a resolution and learn to compromise. Rather than asking yourself: why does conflict arise? You should ask yourself, what solutions/skills have I developed across my relationships to use in times of difficulty?
Open communication is paramount to resolving conflict and finding a willing partner (who is interested in peace) is just as important, too.
I taught my partner how to communicate better and he thanks me regularly for that!
Our relationship is good but we still (like any other couple) have our 'off' days. I just tell him openly how I feel even when I'm agitated and then I LISTEN to how he perceived the entire situation. The problem is too many people ASSUME others see reality through the same lenses. When we stop and listen to how someone else viewed it, your empathy levels naturally increase as you realise (most of the time) it was carelessness and their intentions were not laced with malice. So you forgive and explain clearly what you don't like. THEN you remind yourself that they're only human and they probably will make that mistake again because human beings can just be shytty. But you love 'em anyway because love is about accepting someone's imperfections (exception is verbal or physical abuse of course). However, we're all flawed and you learn how to accept your partner fully and love them on days even when you don't want to love them.
That's what I've learned anyway.
Hope that helps!