Why do people commit suicide when they have things that they care about?

I searched around and figured that this would be an appropriate subreddit to ask in. I hope this is the right flair, none of them seemed to fit quite right.

I saw this post, and it struck me as a bit odd (I hope this doesn't go against the no personal story rule). This person's former girlfriend committed suicide, despite obviously caring about their relationship with OOP; Why?

When I think of reasons to commit suicide, it would mostly just be because of lack of things that you care about, which doesn't seem to apply in this case; This person cared about OOP, they said as much, and yet they committed suicide, meaning they could no longer experience the things they care about, and in fact they harm them - This seems quite counterintuitive.

Any thoughts?