It's easy to be "in love" when you both want the same things.
At the start of most relationships both people tend to bend over backwards to please the other person they're attracted to.
The word "no" is seldom if ever used. Sex is off the charts, laughter comes easily, and life is a bowl of cherries.
The first real fight or disagreement changes everything. Hurtful words are used towards one another, rude behavior, or the "silent treatment" Even though the couple eventually "makes up" the first (boundary) has been established. As more boundaries arise the more a couple notices how "different" they (really) are from one another. Gradually each one adopts a "strategy" for getting along as well as getting what they want from the relationship.
In some instances people simply learn to walk on egg shells and simply "go along in order to get along" rather than deal with fighting.
Overtime the "seed or resentment" grows within as one or both people struggle with not being their "authentic self". If you start to feel that things always have to be his or her way you begin to feel as though they don't care about you. Therefore you start pulling away emotionally and then physically until the frustration causes you to blow up. Some married couples don't spend time in the same room of the house any longer than they have to.
In some instances past arguments and the words/behavior used are never forgotten. Sometimes there has been instances of verbal/physical abuse, belittling, and inconsiderate behavior that causes the couple to adopt an "every man or woman for himself" mentality. "Us and We" becomes "You and Me" until one of them has had enough and decides to breakup or cheat.
If the couple has been together for a long time one or both of them will attempt to win the battle of friends and family to have them choose their side.
After the breakup if one appears to be doing better than the other or seems happier it drives their ex insane.
Having said that "hate" is not the opposite of love, "indifference" is. If someone hates another they're still "emotionally connected" while (indifference) is as if they never knew each other!
People are quick to blame their mates for their unhappiness rather than to look at their own criteria for choosing them in the first place! Each of us chooses our own friends, lovers, and spouse. Until one learns to make better choices for them self they're likely to keep making the same mistakes.