The romantic idea of dating is something. Now, I believe there are other deeper forces at play. Something we could call destiny line. Long term evolution for that person.
When two persons meet and have a relationship, they share and explore together for while. They learn from each other, initiate new things together, etc.
After a while, the potential of this relationship can simply run out of energy. If you take a girl over a period of a life time, she might have to experience along her own inner growth path a series of energies, situations, etc. simply to expand her consciousness.
After a while, what one single person can offer (a present partner or bf, an ex) can simply feel limited for her spirit. Her whole being might be calling for something bigger, further or simply different.
A fixed cluster of energies (like a relationship) can sometimes stop a person's evolution.
You, as an individual can "fight" against that person's destiny line. You can try to whop it, pressure it, manipulate it, but it won't work. You can't cheat with someone's destiny line. Their own spirit knows where they are going and what they want to experience in this life time.
What matters the most is not what you want: it is what they want, and what their own spirit wants for them.
If something essential is missing, they'll keep on searching until they find it. Sometimes no matter how much will power, determination and desire you have, it's not enough. That person's line of evolution is already set and they move in that direction.
The only thing you can do is support this person in reaching their inner goals. Sometimes the best way you can help is simply by being out of the way.
I know, it's tough. But truly, I believe this is the underlying dynamic I see behind what you describe, break ups, etc.
When you understand this type of dynamics, you role and mind set totally shifts.
There is more to say but I'll stop here for now.
Does this make sense?
Good luck and stay in touch
vitalcoach