why +

why +ve feedback is not preffered in op-amp?

In what application? Sometimes it is. You need to be MUCH more specific if you expect to get answers.

When DC positive feedback is added to an opamp then it causes the output to suddenly go as high as it can go or go as low as it can go. It is called "hysteresis" in a comparator circuit.
When AC positive feedback is added to an opamp then it causes oscillation.

Negative feedback causes the voltage gain of one million or more to be reduced to a reasonable amount, increases the bandwidth and reduces distortion of an opamp or amplifier.

"Positive" feedback is good for humans and bootstrap BJT bias circuits improve performance because the gain is <1.

But if feedback gain >1, they latch up. If there is also latency, they oscillate.

+ feedback is used for an oscillator, there are probably numerous other circuits too.

POSITIVE F/B whether is systems or op amp the effect is the same output saturation to one rail or the other depending on polarity input.