Probably because Louise and Haru were together before, and their relationship is as deep and meaningful as Legosi and Haru. And Legosi would be hurt if they got back together, but at the same time being the lovely giant he is, he would be okay with it after everything that has happened.
I don't know why...... but when I see louise and haru together i become very pissed (╬ ̄皿 ̄) and start hating haru. ( but i like shiping legosi x haru )