@WantedLife My own reason... I broke up with my first serious girlfriend (we were in our early twenties and had been together for almost two years) because I knew that she wanted to go and live/study abroad for a while, even though she would not be doing that for at least another two or three years probably younger, and I knew I would not be able to leave my home and family/friends to move abroad too. In some twisted broken logic part of my brain I thought that I was better to end things sooner rather than get even closer than we already were and go through even more heartbreak and pain when the time came to separate when she moved abroad.
Of course we were both devastated anyway, and nothing is permanent. Making things work long-distance and still seeing each other over holidays never even occurred to me; she did eventually move to Italy for a while but was always coming back, and she has now been back in England for a few years. We would probably have found a way to make it work, although I have no regrets. I still feel sometimes it was a daft reason.