In the case of Zack, insecurity.
He's afraid that the feelings aren't mutual and that by confessing his, he runs the risk of making things "weird" between them and inevitably losing the friend he already has. So he'd rather settle for what he has than risk losing it all, even if it means he won't be any happier for it.
As for Kyara, unresolved guilt.
Some events in the past, I will not delve into deeper for spoiler reasons, caused a lot of harm and she feels responsible. She fears that growing closer with the person she likes will end up causing them more pain, because she believes she has hurt them before. Now while she claims to keep her distance, she has done pretty much the opposite of that, and as events unfold in future chapters this guilt will flare up immensely! Especially as things go less and less her way and is less in control.