While Hijikata knows that Eren and Reiner have something going on, he still doesn't understand how Eren could look past all that happened in Paradis Island... but with him slated to learn that his poor innocent babygirl is going to end the world, maybe he could also stand to catch a glimpse of ReinEren, if you're up for it!

I reckon only the latter options would really change his understanding, so perhaps the mutual confession of wanting the other one to survive? (That definitely won't be awkward later!)