In the Novel they explain better the relatuonship between Song Yan and Yu Luotong (The guy he had a crush on in his previous life). Spoiler Alert.
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In the previous life Song Yan felt that he needed to protect Yu Luotong, and that his love was born out of that wanting to protect. He told Yu Luotong that if something happened to him that he would also die. So when Yu Luotong was presumably dead, the internal guilt that Song Yan felt about their promise to protect each other and live/die together kept him from being a loving person to Xia Lin. Then when he found out about the truth he realized that he actually loved Xia Lin, who was the only one who always stood by his side.
Of course it doesn't excuse his behaviour, but i think that he gets a redemption arc, because he truly shows his love to Xia Lin BEFORE AND AFTER he regains his memories in the second timeline.
I don't like the ML lol I've already read up to 8 chapters in other sites and the ML's ex is their classmate and the ML being the asshole that he is, is already flirting with him lol