What is the crucial ingredient for the formation of lightning such that without this ingredient, lightning does not form in a thunderstorm?

High Potential Difference

Explanation:

Lightning is a type of Electrical Discharge and for any electrical discharge to happen we require a high potential difference between two areas in space.

In case of lightning this potential difference can be formed between 2 clouds, between a cloud and the ground or two different areas in the same cloud.

So during lightning this potential difference causes different type of lightning due to electrification followed by charge separation and hence aggregation of charge is specific areas of clouds.