i have an idea. what if Stannins sent the Pink Letter because he knew that he needed more men? i mean, Jon always told him the Nights Watch is neutral but gave him shelter and advice and obviously hates the boltons. Stannis knows or hopes that Jon cannot live with Arya being Ramsays wife. it could be possible that Stannis wrote the letter to provoke Jon as hard as he could to make him come and bring as much men as he can gather, because if Stannis just wrote " dear John we could use some help here" Jon would go "sorry i have to stay neutral".
