What could be the reasons why long term relationships like five+ years end?

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When I heard about this news about my friend, I was really shocked, I thought relationships like that would never end because they've been together for a long time and their relationship was somewhat tried and tested through time. I really don't want them to get separated but when I heard the girl's reason for breaking up I somehow agreed. Her boyfriend is very controlling. I don't know, I just hope they still get to resolve this thing because 5 years is really something not to be wasted.