Frankly, by themselves there's nothing wrong with them. It's the fact that most of the reasoning behind working a curse or a binding is typically petty or born out of knee-jerk reaction. Typically the matters that one would seek to resolve via such workings could be resolved through less effort and less negativity through other, common sense means. It's the same reason that the average person shouldn't carry a weapon (albeit, likely with far less drastic consequences). Anger, jealousy, or any other such petty emotion may leave an individual resorting to that weapon when words, avoidance, or other more logical, less damaging means of resolving the situation might be utilized. This is even more true when reacting to a situation, such as the actions of an ex fiance, where reason and logic become overruled by emotion. We've all been there. We've all had a moment where we wished someone harm because of how they made us feel. But the bottom line is that while it was their choice to act in a hurtful manner, the way that we react to it is our choice. We can rise up, be better people, learn from the experience, and otherwise remove the influence of the negative individual from our life in a less volatile manner (no drunk texts, no late night phone calls, removing them from our various contact lists, blocking them if they keep contacting us, or otherwise avoiding the person in general). If it's not ok to react by punching someone in the face or otherwise doing them harm, why is it ok to turn to magical means to do harm? Sure, it feels right at the moment, but it's not going to solve anything and at the very least you're going to bruise your knuckles, or maybe even break your hand.
Further, most of the time working a truly effective curse requires that the one be in a state of mind in which you are solely focused on doing harm to another. This state of mind is not exactly a great place to allow one's self to be in with any frequency, as it starts to be comfortable. When you're comfortable with doing harm to others at any given point, the idea of doing so becomes the knee-jerk reaction and so does the action that follows. The negativity involved in such a frame of mind also becomes habitual, lingering in one's mind and otherwise impacting one's life for the worse. Before you know it, you're viewing everything through a negative light and you're that jerk who everyone hates because you're quick to snap at everyone over nothing, reacting violently over nothing, or otherwise being melodramatic.
Working with curses or similar magical concepts, in my opinion, is typically best reserved for a sort of self-defense situation, with cold calculation, when all other options of have been exhausted. And mind you, I don't mean self defense in terms of bullies in life or physical altercations. Again, there are far more efficient, common sense methods of dealing with such situations.
In terms of binding, the results often have unforeseen consequences that one may not want, impact the individual the individual working the binding or others whom one did not realize would be caught up in the situation. It's a silly example, but the movie The Craft shows some fluffed up Hollywood examples of various bindings/curses that are used by the "witches" involved, that go way, way out of control. Obviously magic and like concepts are not as depicted in the movie, but the idea of things spiraling out of control remains closely the same when ever one attempts to use a binding, curse, or any means to influence others.
Plus, as will no doubt be said repeatedly, there are various concepts of what goes around, comes around. Karmic retribution, the law of three, and other such things are often believed to impact you if you seek to do harm. That, however, is up to the individual belief system and I've yet to personally witness any sort of cosmic give and take based on this ideology that didn't seem to be more than random chance.
Plus, in the situation you describe, reminding someone of what's most important (you), is quite situation and selfish. Everyone is the protagonist of their own story. That doesn't mean you're not just a supporting cast member in someone else's. Attempting to force yourself to be such won't help anyone, least of all you. Even if such curses and bindings worked (and they rarely do when most people cast them, to be honest), an individual so compelled would have their subconscious and other such parts of their mind still working as they normally do. They'd be torn, and on some level be resentful to you. What starts out sweet and nice and feeling like someone worships you will turn into a situation where they are profoundly unhappy being with you, possibly turning everything into an argument or a fight, or simply being miserable in the life you are effectively trying to force upon them. Would you rather not just have someone choose to see you as the most important thing? That seems like it'd be far more pleasant to me.
Hope that's a better answer than "curses and binding is bad, mmkay."