Usually, when this happens, one person still wants to be with the other and so they agree it is okay to be friends, when in fact they want more than friendship. Since this is not an option, that person will take what they can get, hoping to rekindle the relationship once again. If this is the case, there will be arguing from jealousy and disappointment.
If this isn't the case - it seems you just don't get along very well as friends when there isn't intimacy involved
