Almost any story you write will involve character relationships. These relationships can be familial, between friends, romantic, business-related, etc. Often character relationships are a good source of conflict for your story, even if only as a subplot.
I've had a theory for a long time about personal relationships, which I call the Three C's. In order for a relationship to be functional and healthy, you need Communication, Cooperation, and Compromise. Note that in order for the Three C's to benefit a relationship, all participants in the relationship have to reciprocate, or at least try to. As an author, you can create conflict by choosing to have your characters neglect one or more of these important principles.
Communication is about, well, talking to each other. Keeping in touch. You can't have a healthy relationship if you can't tell each other what's going on. (Yes, this also involves listening.) A character who, for whatever reason, can't or won't talk about their feelings or situation, who doesn't listen, or only hears what they want to hear, invites misunderstandings which can lead to conflict.
Cooperation involves working together toward a shared goal. How many movie heroes have gruffly muttered "I work alone," only to end up needing someone's help later in the show? A character who stubbornly refuses to be a team player can work against their own self-interest in all sorts of fun ways.
Compromise means that sometimes you don't get your own way, and the same for the other person. Compromise can be anything from finding agreeable third options—"You hate Italian, I hate French, so let's have Mexican for dinner!"—to splitting up distasteful chores—"you sweep this week and I'll sweep next week"—to agreeing to disagree—"I really hate that band, so why don't you go with Bob? You and I can go to a movie tomorrow." Of course, to create conflict your characters must insist on doing things their way, getting what they want when they want it with little or no regard for other people's wants or needs.
By keeping the Three C's in mind, you can create simple and believable relationship conflicts that can be easily controlled and shaped into important, relevant, and satisfying resolutions.