Am answering this in a general sense.
Most of us perceive "to love" and "being loved" as good.
In Buddhism, without going into talking about Karma, it is an
You love someone, more or less on the condition that, this person must love you back. If not, what would happen and how many relationships have ended up in tragedy.
Again, am not going into the Karma part of this.
Just speaking in terms of attachment.
Why do we love someone. Again, very generally speaking, first comes into play is the "sense of perception" (i.e. "She is beautiful", "He is just gorgeous, look at his body", etc).
Then, based on that, you conjure up thoughts and feelings that favor and fulfill that "sense of perception" ("No, no, no, he is not bad, I still think he is a good person", "I dunno why, even though she lied to her own mother and has stolen some money in the past, I still think she was just mistaken; deep inside, I think she is a good person. She is just lost and having a bad time in the moment. I think she is nice. Give her a chance").
Again, speaking very generally, you would do whatever to please her /
him, doing things that you wouldn't normally do - that's one suffering
(a debt, if you will).
Then just to please her, you try to lavish her with items you don't normally buy.
For example, a leather bag, leather shoes, a gorgeous diner, etc etc. Think of these actions, behind these, someone / thing had to suffer / die for you, for your fulfillment of pleasing her.
Again, putting Karma aside.
Let say you are normally a very calm person, knowing how to avoid conflicts and ease things. But since you are with a girl now, you need to become / become competitive and more aggressive (i.e. if she complains that a guy at the train station is looking at her, you would need to stare at his guy back, when normally if you were being stared at, you had your own way to avoid conflict and stay calm).
Attachment stacks up, likewise sufferings.
And so, let say you still decided to do all of the above and is OK for you, then if one day she so happens to do something that doesn't please you back, all of what you have done for her rush at you in thoughts and your anger arises ("You know I love you, how much have I done for you?, I I I, did this; when I was sick, I still went to you and I I I". And she also "I I I".).
From now from, it cycles further.
Then, at the very very last thought before you decided to leave her, your first sight (the beauty of her) comes to "conciliate" your decision of leaving this relationship. And it goes on like this.
Let say somehow it turns out OK and you and her have kids. It is another attachment stacked up.
You need to raise them. The stress, etc etc, forces you to do something you don't normally do and against your will.
Again, putting aside Karma and be very general:
You need to fulfill your kids' needs (food, toys, clothes, iPad?). And now you need more money. Normally you work and that's it, but suddenly money is important and thus your job and a raise; so, you start office politics and ooze out some co-workers, who you think might endanger your position). Sufferings to all parties including yourself. Debts.
It goes on and on.
Your kids, who you tried so hard to put into Stanford, turn out to be someone who smokes and drinks on the streets with a guitar, living day by day by stealing. Another attachment that causes sufferings here - your "kindness" to your kids are not reciprocated. You disown them, and they in turn off and on shout at you, etc etc.
Or: they turn out to be graduates from Stanford. But not visiting you enough afterwards, not saying to you enough "thank you, papa" and your anger arises.
All these are "binary" - your love depends on the others', the others' yours.
It will be payed out and must be re-payed.
All of the above begin with one perception (re-cap: "He is gorgeous", "She is so fine, so pretty").
The cycle "cycles" onwards for generations.
So, if you are already in a relationship and would like to know if a "divorce" would be frowned upon, the answer is, all has already begun from the "first perception" that led into "loving" and "wanting to be loved" with this person.
If the thought "why is loving another so 'bad'?" or "I think loving and being loved is a most beautiful thing!", then please re-visit my answer here someday in the future as you continue your path with Buddhism and I hope to hear from you then.
I am not saying / defining here some thing is bad; rather: not to attach oneself or get attached to that something is good enough.
Roses grow, fish swims, but we don't need to pluck the roses or catch the fish and put it into a tank. Let them be where they are and you so likewise.