Remus internally tweaking out when Mary and Sirius were together because he thought they liked each other and he "never got what he wanted, not like he deserved it", but in private Sirius would talk Mary's ears off about Remus. Mary had tried her best to drop hints at Remus of Sirius' feelings (pan camera to Sirius biting his lip anxiously watching Mary and Remus interact) but Remus is just so oblivious when it comes to Sirius and all he saw was that Mary and Sirius were together. It took Remus some time to work out that Mary is just very affectionate once she's comfortable with a person because he found that she behaved the same with him and a few of their other friends... it took him even longer to work out that Mary is indeed aroace... anyways. Mary slapped some sense into Remus and Sirius both.
