I don't believe that your friend left the friendship because he felt like relationship is not good for you...
That friend either likes you and probably has not found the suitable time or way to too you so something in between
Or maybe your friend has a mixed feeling about his feelings for you...
Another reason could be that your friend don like the kind of person you are dating...
A whole lot can be involved but please value your friends also respect your relationship...
In fact, I don't know what's going on in his head, maybe he needs some time to think about his attitude, I hope that in the future he will have a different opinion about this situation.
Hope so