The thing you need to understand about Robb/Sansa as a ship is that they both had expectations about their lives and they were both horribly wrong, and it devastated them. Being a king was nothing that Robb expected, and being a lady was nothing like Sansa dreamed.
But the expectations they had are personified by one another. Sansa dreamed of lords like Robb and Robb dreamed of being someone like the men in his sister’s songs.
Thank you.