First off, I want to say that 'supposed to' is a very strong phrase. If this phrase ends up in your thoughts or perceptions, it's usually a good indicator that what follows it may not be a part of your authentic feelings about what you as an individual believe in.
This article makes some excellent points about the fact that there are several myths on which we as modern day society base our expectations of ourselves and our partners in a marriage or long-term romantic commitment. The most striking of these myths is that oftentimes, two people think that if they love each other, they shouldn't fight.
The reality is that any two people who spend enough time together will eventually find themselves in some sort of conflict. This reality has no bounds as far as which two people you consider. This could be with a family member, a lover, or a child. Just because there is conflict doesn't mean that the love has come and gone.
Conflict is a very natural and without it, we wouldn't have a forum to exchange differing ideas. Note that there are a plethora of ways in which conflict can be had productively and kindly. There is never a good time to go ballistic on anyone. Learning to manage conflict and to repair the possibly ruptured connection afterwards are key.
As you can read more about in this article, Dr. John Gottman's research found that a whopping 69% of the ongoing problems in marriage are unresolvable. This statistic may be comforting to many people. It means that the majority of couples are having the same difficulties you are – about money, sex, in-laws, kids, whatever. Stick with it, and you will find that choosing to move through time with a fellow flawed human, learning and growing with somebody you love and trust is, despite all the difficulty, is what really makes us happy at the end of the day.