I'll make it even cuter. Charlie and Riley have been friends since they were kids and she's been in love with him since they were teenagers. It was a night spent on the beach after one of his visits home (he joined the military after high school) that made him realize he wanted to be with her. So when he left the military for good, that's when he showed up in Boston. Charlie's always been her person, he was just a little slow to catch on 😂
Riley would've attempted the same. Maybe they were neighbors at one point while living in Boston and that's how they met? She lived in several different places before moving in with Charlie.