I think that if you really like someone a lot in that way, but they don't feel the same way towards you its unhealthy to maintain a friendship with them. Because a friendship will never really be what you want to have, you will always want something more with them. The more time you spend getting to know them the more feelings you will develop. You will constantly be hoping they might feel differently about you over time, but sometimes it's just not meant to be unfortunately. If you continue the friendship, over time you may have to come to terms with the fact that they may begin to date other people and talk to you about them which is the hardest thing of all. Ultimately it is best to move on entirely if someone doesn't feel the same way about you as you do them. Try to have as little means of contact and reminders of them as possible. It will initially hurt but over time when you meet someone better it will seem like the best decision you could have made.