Why did he have to cut me off completely just because his ex came back into the picture? Why can't we be friends?

Everything was going great between us and then suddenly he cut off all communications with me, I later found out his ex came back into his life (they were on and off for 6 years for 3 years of their relationships few years ago), apparently still kept in contact sometimes.

He ended it too.

Why couldn't we have just gone back to being friends, why does he have to cut off everything with me?
Why did he have to cut me off completely just because his ex came back into the picture? Why can't we be friends?
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