Congratulations on that! Dream come true for a writer.
To answer your question: Well, I guess it goes to motivation. I've never really seen it as a stress-test of the relationship, as you put it, but I suppose some parts of it can be taken that way. My interpretation is more as follows.
I think there are some instances where people are scared of vulnerability. I'm very private, slow moving, and generally guarded in the lead up to a relationship, as someone who is pretty clearly dismissive-avoidant in their attachment style.
So, in the relationship progression with the last woman I had something serious going on with, for example, things were fairly casual for a few months but eventually, I wanted to show her how much I liked her, so I would start trying to do things to that effect.
She mentioned at some point that she hadn't ridden a horse since she was a child, and she missed it, so I picked her up one day for a "surprise date" and took her horseback riding.
She seemed totally weirded out by it. Maybe that's to be expected as I was pretty nonchalant up to that point, and it was the first instance of me showing more initiative towards her.
But then I'd do other things after that: We both liked art, so I took her to an art museum. I planned out dinner dates. I got her small gifts. I started opening up more verbally.
Every time I'd do this she'd seem uncomfortable and usually a petty fight would start in the aftermath. Mind you, when I was blase about the whole thing, just showing basically zero emotion by way of effort or word, she was all over me. She'd drive over to my place damn near every day, she was saying how she didn't want to see anyone else, etc.
Yet, every time things progressed, or I tried opening up, I felt like I was punished for it. We eventually came to live together for a short time, which she heavily pushed for, saying "I'm here all the time anyways". But after a month or so I had to ask her to leave because she was outright disrespectful to me--insulting my career and station in life--and then refusing to apologize for it.
She had never been insulting to me before that. And, in fact, after I asked her to move back out, she then went back on her initial refusal to apologize and formally did so.
I imagine in her case, she was something of a dismissive-avoidant as well and was quite uncomfortable in the relationship when things started to get too real. So that's a case of fearing vulnerability, but I believe there's an even more prevalent cause.
In general, I'm of the mind that most people have a preferred hierarchical position in any relationship. A lot of the women I've dated like to be submissives (and I don't mean this in a sexual way). Fundamentally, they want to feel like they need their man more than he needs them.
I think these people have an anxious attachment style, and their love only feels real when they're a little bit scared of losing it. Now, when you move to commitment with this type, you're letting them know that you really want to be with them.
This means they have emotional leverage over you, they have a sense of control, and they start to feel safe. Well, since so much of the intensity of emotions that they normally feel in a relationship is derived from fear, they now feel less, and they feel uncomfortable being in a position of equality or dominance.
They tend to lash out. In general, hierarchy flips are very uncomfortable for people. Again, the effect is that you get punished for commitment, as the commitment itself undermines the desired dynamic.
If you're interested, I wrote all about the "two-person hierarchy" here:
Now, you might be saying "well then find someone who isn't dismissive-avoidant or anxious in their attachment style." Now, I don't know if I can only attract a certain type because of my own attachment style, but if I were prone to making definitive statements based on my own experience, I would've said the above applies to all women--as it's all I've ever seen personally.
In my view, it might even be true of all people, but I don't date men so I would never see that directly.
But either way, the end result is that I believe that for one of the above reasons, it seems to me the relationship will become fraught with issues because of full commitment.