I'll chime in briefly.
I tend to be one who leaves forever, but I also tend to be one who doesn't know my feelings right away. This has led to problems from the beginning that I could not quite identify, just an unsettled feeling.
Additionally, it's often difficult for me to express how I'm feeling and this makes me personally feel on shaky ground. So the uncertainty builds until it's no longer underneath the surface. Consequently, once I finally figure out what I'm feeling and my ground is steady, that's it. That's the "spontaneous" quality you mention. For me, it isn't spontaneous--it's just not immediately apparent. And I insist to myself that dedication and understanding will solve any relationship problem, which means that I often ignore that unsettled feeling to the detriment of everyone.
Makes relationships a minefield, at least for me.
The few partners I have been with have ranged from manipulative jerks to loving and supportive, so it's not just an unfortunate choice of partner. Also, breaking up causes me great pain, which I am also not able to really show. I've been called "cold" and "callous" in the past.
For myself, I have shelved relationships for the time being until I know my feelings more immediately, and until I can express those feelings in the moment. Until then, as painful as it may seem, the most compassionate move I can think of is to walk away. And I need lots of space to process the ordeal.
I agree that we're all different, but I hope this helps.