People are mad about Sera but she waited for her boyfriend(the MC) to come back from service. He doesn't contact her at all when he comes back and the first time they meet is when he's with the younger version of his mom and being secretive about it and looked like he didn't care about their relationship anymore(because of his shortened lifespan but he won't tell her about it, think about this from her perspective).
So to anyone who isn't omniscient it's pretty much like he cheated on her and brushed her off like they never even knew each other and all of the incidents revolve around his mom.