- I want to spend the rest of my life with you.
- I know we'll be together forever.
- We're going to make this relationship work, no matter what.
- I love you.
Those are the kinds of things Willow and Terra said to each other. Their relationship was so perfect, and they knew it. They'd only ever had one or two fights.
Terra left within a few days. How did it happen? They fought about Willow using too much magic. Then Willow used a spell to make Terra forget the fight. Terra was pissed when she found out. When Willow did it again, that sealed the deal. The end.
They didn't talk about it much. They didn't give it a few years (only a small part of their lives) to make sure they weren't throwing their perfect life together away. Not even a few weeks to get perspective on the matter.
And problem solving? What did they do to fix it? Nothing. Willow had a "no magic for a week, then I'll reconsider" ultimatum that she blew. That's very harsh pressure. And not very helpful.
But anyway, consider this: What is Terra's excuse for going back on her relationship promises? That Willow did something horrible. But Terra made those promises to someone capable of doing horrible things. That was her mistake, not Willow's. Willow didn't lie, trick, or mislead Terra about who she was. Well, maybe she did a little -- that is expected in romantic relationships -- but nothing relevant. Quite the opposite: Terra was catching on to Willow's magic-habit right as it formed, because Willow was so open about it.
When people say "forever", they mean, "I might leave you tomorrow if I subjectively feel betrayed or upset enough. I might make no attempt to fix things, if it seems too hard to do so. And I might blame you for everything. And the chance I'll do this is not 0.0000000000001%. It's more like 0.2% per day."
a few days later:
Willow: it's better this way
a bit later, Willow gets much worse (magic=drugs parallels). Buffy finds out. she has a different reaction:
so remember terra's was: omg you aren't perfect? you betrayed me. bye.
Buffy's is: omg you nearly got my sister killed? well, i'll help you now. we'll get rid of everything that might remind you of magic from the house. even though the sister you almost killed complains that she likes some of the stuff being got rid of.
so, lovers leave, friends help. that's what all those promises to stay forever amount to.