Oscillating system via negative feed back?
The answer is relatively simple:
Each linear oscillator needs a loop gain of (at least) unity (unity magnitude and phase shift of zero deg) at one frequency only.

That means: We need a frequency-selective circuitry which can meet this condition at the desired frequency.

If we want to use a fixed gain stage (which is not always the case, we also can use integrators) we have two choices: inverting or non-inverting.

1.) Non-inverting (phase shift zero): The passive network must produce a phase shift of zero at the desired frequency (example: bandpass).

2.) Inverting (180 deg phase shift): The passive network must produce -180 deg at the desired frequency (example: three RC lowpass stages). 

Answer to your final question:
Why do we need positive feedback to implement an oscillating system ?  

In order to avoid misinterpretations, I think that we should say: We need always a positive loop gain (of unity or - in practice - slightly larger) at the desired frequency. More than that - at the same time we need negative loop gain (negative feedback) for DC (stable bias point).