It’s because he wasn’t straight forward with her about how much he was feeling Clarke, he kept leading her on in little ways like bringing them both breakfast and saying he still wanted to pursue O’s connection even though he was telling everyone else otherwise and being kinda shady behind her back
If you rewatch the little things he did it was obvious he was gonna choose Clarke and was leading O on. His half assed lowk rude comments towards O plus the way he was trying to hide still being physical w Clarke after O was back in the villa but didn’t actually stop to be respectful of the situation and at the same time he was telling O he’d still wanted to explore their connection