Mutual feelings or not at all. If you like someone but they don't like you how long can you continue this relationship? It will only wear you down and make you disappointed. However, even if the best and most perfect person in the world likes you but you don't hold the same feelings, isn't it cruel to stay with them and make them think you like them?
Mutual feelings and most importantly mutual respect. There is also nothing wrong with not being with anyone, after all, if you can't be happy by and love yourself how can you be happy with someone else?